I think personally I see DL fights as similar to FE arena fights, and would thus apply their general mechanics as best as possible to the environment. At least based on a quick test in Shadow Dragon, arena combats work by repeating player-phase attack until one side dies - in other words, the enemy never gets its own turn. Now, obviously, taking this literally is a bad DL interp (it fucks over anyone who has options), so I can't take it 100%, but I can use it to guide my interpretations.
For the interested, the way I came across the testing result is as follows. I sent L3 Navarre, with 12 spd, into the chapter 4 arena, and with savestates got a test on both a battle in which he doubled the enemy, and a battle in which he did not. When he doubles the enemy, the sequence of actions is Navarre-enemy-Navarre-repeat. Extrapolated over a long battle: (N-E-N)-(N-E-N)-(N-E-N)... When he does not double the enemy, the sequence of actions is Navarre-enemy-repeat: (N-E)-(N-E)-(N-E)... As such, it is clear that Navarre is always the one initiating a combat round at all times, and the enemy's interaction is limited to counterattacks.
Counters
In short: no. I think FE characters should either get to counter (and be countered), or get to double (or be doubled), but not both, and I like them getting doubling/doubled more, because I think that comes closest to the arena-like conditions I see DL fights being while still allowing both combatants to actually have turns.
If I were going to consider counters in FE vs. non-FE duels for some reason, I would consider anything that deals HP damage to be counterable (with considerations made for attack range); yes, this includes MT, I do not feel that MT attacks (which are nonexistent in Fire Emblem but moderately common in other games, including other SRPGs) should effectively be a "get out of being countered free" card. I would also consider anyone fighting against the FE PC to counter according to FE rules.
But that all said, still, the only case I'd normally consider counters at all would be in duels between two FE characters, because I would consider such duels as if they were being fought in the FE arena, with all the mechanical considerations that entails.
Doubling
I basically agree with SnowFire's interpretation for doubling as written, except that I'd use a consistent standard of 4 points of speed difference regardless of the FE game involved in the duel. (why 4? because more FE games use 4 than any other threshold.) I'd also be a bit more lenient on what was allowed to double (IMO, basically anything that inflicts damage should be able to double), but if a non-FE PC gets to double an FE PC, they have to do the same thing twice; they can't, say, attack once and heal. In short I view the non-FE PC that doubles an FE PC as still only getting one discrete 'turn' with the proviso that a damage-dealing action they take on that turn fires twice; if they take a non-damage dealing action, they don't get to act twice. This probably disfavors fast characters with healing but poor damage, but whatever.
Initiative and speed
Agree with SnowFire's interpretation full stop here, in line with my arena interp. For duels between two FE characters, I'd arbitrarily give initiative to the character with higher scaled AS.
Other considerations
On EXP interpretations: I generally favor equal EXP if it's available; characters with bad availability should have it count against them. In the specific case of FE13, this includes Second Sealing to the same class they were before upon hitting level cap. (More on this in a moment, though.) SnowFire has valid concerns about the implications of equal EXP on projects; IMO, the best solution here is to attempt to apply whatever sort of EXP scaling the FE game involved has (if there is any) - most FE games will scale EXP gained by a higher level character downward, so account for that. I also wouldn't take into account whether the PC can actually win combats in-game for an equal exp interp; one assumes that if the player were painstakingly going out of their way to make sure everyone gained EXP evenly, they'd also painstakingly go out of their way to set up situations where the underleveled PC could safely gain their fair share. I could support selecting a level floor for the EXP-screwed, but their poor availability should still be taken into account somehow.
On evasion: I prefer taking FE characters' evasion as a relative quantity rather than an absolute. (In other words, figure out the game's evasion average and extrapolate from there, like Malcolm said.) As for applying it, evasion in FE applies to basically everything offensive, whether it be physical, magical, or status, and I see no reason to deviate from that, so all non-FE attacks should always check against FE evasion unless they are explicitly ITE.
On criticals: FE characters absolutely should get criticals if a duel goes long enough for them to have the chance reasonably, but I probably wouldn't consider them in averages. Characters from games with no luck stat should have luck equal to the enemy average in whatever FE game the FE dueler came from. Characters from games with a luck stat should use it (scaled to FE numbers in the same manner as speed was for doubling considerations).
On branching promotions: I favor allowing class switch by match, definitely, for games where this matters (FE2, FE8, FE13). The same extends to asset/flaw considerations for Chris (FE12) or Robin (FE13). That said, this probably doesn't come up often, most of the time one or the other class is clearly superior for DL usage.
On laguz: I don't vote on FE10 and don't really use laguz in FE9, so no opinion.
On excluding people from averages: I would exclude people who either A) can never actually attack in-game (e.g. Ninian) or B) the character is explicitly excluded from nomination for some reason (as has been suggested for FE9 non-royal laguz due to interp issues). Basically, if a PC has even a remote chance of ever being nominated into a duel, they should count towards the averages.
On FE11/12 reclassing: Definite no. If FE12 ever gets a stat topic for some reason, Chris (Avatar/My Unit/whatever) should be taken as their default class (paladin).
On FE13 reclassing... I would actually lean toward allowing it with an interpretation that goes more or less as follows:
- nobody reclasses before the end of chapter 16, so equal exp figures to that point are critically needed
- standard PCs reclass at the end of a chapter in which they reach 20/10 or 20/15, if at all. if they reclass at 20/10 they should probably be figured to get a C weapon rank in any weapons their original class didn't have; if they reclass at 20/15, they should be figured to get a D rank in weapons their original class didn't have. Unless we exclude brave weapons, there are probably not many PCs that actually want to reclass under this interpretation; this is fine, in my opinion.
- PCs who don't promote should be allowed to reclass once immediately at the end of chapter 16 provided they meet the minimum requirements for doing so (level 10), if they choose to do so, and should have stats calculated for each possibility accordingly (with them promoting at level 20 in their new class). if they do not reclass immediately at the end of chapter 16, they cannot reclass until level 30, and then can only reclass to their original class again. in any case, such PCs should probably be considered to have a B weapon rank in any of the weapons available to their base class and C in anything exclusive to their promoted class. Exception: Tiki, since she joins later than chapter 16; if she wants to reclass out of manakete, she has to do it immediately on join before she gains any EXP, and she probably gets one less weapon rank (absolute) due to the time considerations (when I say 'absolute' here I mean one total, not one in each weapon). that said, I'm pretty sure the only non-promoting PC who actually wants to reclass is Donnel, as the others likely want their stone bonuses.
Last but not least, on game forms: if someone actually figures out how SpotPass PC growths work, I would actually be amenable to considering FE13 forms of previous FE games' characters with FE13 equipment. any such PC would be taken at 20/20 with no reclassing, and would be compared to FE13 averages (but not included in them).