1. Yes, yes, we know you scale AS differently. Although, far be it for me to remind you about things about your view, but if you scale AS into damage you should scale it into durability as well (Last Hope swordmasters will certainly double the Garcia-tier speed PCs, heroes/mage knights may as well), making Amelia tankier, and... this is actually very problematic for Feena, see below.
2. I was assuming 1 def = 1 point of damage taken, yes. If you have more insight into how G1 Def works feel free to say! But I definitely remember the stat mattering, the def differences were considerably more notable than the HP differences. Granted, it was usually modifying an average boss damage of like 25-30% PCHP much of the time, of course, because Grandia 1, but still...
2a. I'm much more likely to close the Def/Res gap to zero than I am to expand it. The final chapter has a higher average enemy Res than Def (as does chapter 18 and probably 16, i.e. half of the final six maps), and almost every boss after your army promotes has more Res than Def, with the singular exception of Morva.
3. I used your stat topic's default averages for Grandia to calculate Feena's pdur.
For what it's worth I think Amelia may win even if she misses the 2HKO. She will still 3HKO with Iron + Iron + Silver (this almost kills average), and she shouldn't be doubled that way. While only 3HKOing gives Feena the ability to heal, I don't think it matters because Feena's damage positively drops off a cliff after the End of the World + two Zap Whips, and Amelia is notably average average pdur, and Feena can't heal very many times, nor is she fast enough to doubleturn at any point (in the CTB sense).
EDIT: Also, while not terribly relevant to the actual debate, I was curious about (4). Amelia with only storebought weapons can one-round wights, archmogalls, gorgons, and anything unfortunate enough to be unpromoted. She misses deathgoyles, maelduins, elder baels (so do all physical fighters), gwyllgis, and cyclopes (so does everyone else except bishops). She misses deathgoyles and dogs due to her speed, the rest due to her power. Is this below average overall for an FE8 unit? Yeah, I'd say so, but my views reflect this by having her have averagish power if she isn't afraid of doubles, and considerably below average damage if she is (and remember, average speed doubles her with silver, to my views). Note also that I don't respect her crit, which actually gives her an okay chance to kill some of the enemies I listed.
I was noting AS differences who might vote otherwise since it's easy to go on autopilot (See me voting Odie over Rose in a past tournament, then switching my vote retroactively after it ended when I realized that I held a viewpoint that the consensus wasn't). I've always factored in HP proration, but it's a pretty paltry effect overall (only 3 PCs have a negative double rate and it's by small amounts). For me, Feena can put herself over the top with the L3 Fire magic.
Regarding defense versus Res, in chapters 15-19 you face 279 enemies, I believe. They average 8.3 Def and 4.2 Res.It's true that bosses are heavier on RES, but ones you would expect to have much higher Res are actually pretty close (Caellach is +2 Def, Valter is +1, Orson is +3 and Morva is +6 thanks to his weapon. So it's definitely not universal). Lyon is -2 Def in one form, -3 in another and Riev is -2 on both. The Medusa is the only boss with notably more RES. I weigh bosses 25%, but that still only puts the discrepency at -3 overall. Now if a lot of DEF tank enemies are able to be smashed by fighters with special weapons, that's definitely a consideration (and let's face it, a loser like Riev getting a 25% credit is pretty nice overall as I remember him being one of the least scary things on C19 by far). I'm guessing you mean the weaponslayers of C17?
C16 has about 1 point of defense over res on average. C18 has a whopping 11 enemies? Its effect is extremely small.
Of course, I might be underrated Amelia against endgame enemies...or more likely I'm overestimating their hideous speed. Regardless, my respect for her is pretty low (Granted, this might be because I've never recruited her as she has a nasty tendency to kamikaze on her recruit map (for me at least)).
As noted, I would be surprised if Grandia defense was worth a lot. I don't really remember Feena taking 25 extra per physical attack at endgame in comparison to Justin (which would be really noticeable). 12.5 might have been possible. Grandia buffs do suck, but it's because you could stack them 5 times. Assuming that the 10% boost is correct, 5 Diggin's is 50% Def up. If Grandia defense was 1:1, that should basically be nulling like all physicals (since Mullen doing 70 is...really high for Grandia). I would suspect that it's about as good at G2/G3.