Kill points:
Some games have durable enemies. Some games have frail enemies. We do not, as a general rule, use their HP to determine how much damage PC casts do. Otherwise, you'd get cases like Grandia 2, where the entire cast has OHKO damage (G2 endgame HP average is about 3000) and cases like Xenosaga 2 where the entire cast would be Punies. (Or, to choose a less extreme but more DL-relevant case, the entire Tales of the Abyss cast would be pretty wretched on damage, as I remember randoms hanging around the 30k-50k region in the final dungeon.)
It makes most sense to compare casts to each other. If I do more damage than average PCs in my cast, I do good damage. If I do less, I do bad damage. This strikes me as personally very intuitive, and to a lot of others in the DL, which is why we adopted it.
Now, why does 2.5x average damage kill 2.5x average durability? The answer is: No reason at all. It is in fact completely arbitrary, chosen simply because some of us decided that made for the most interesting balance of matches. By choosing a higher kill point, you'd lower all damage, which would cause healers and status attacks to dominate too much (even an outstanding damage-dealer can be beaten easily by a healer if you set a kill point of 5x average). By choosing a lower kill point, you'd raise all damage, making OHKOs more common and turning more matches into quickdraws, where alternate strategies would be less effective.
Now, if you take issue with 2.5x, and want to use something else? Terrific! Please do so; use whatever you feel makes most sense to you. All I ask is you be consistent about it. Use 2x average for one cast? Use it for all of 'em.
On Tifa's limits:
Usually, when moves require skill to be pulled off, we assume the player has that skill. We don't assume Yuri misses Judgement rings, even though it's easy enough to do on some of the high-end skills. We don't assume Sabin misses blitzes, even though I certainly botch a Bum Rush now and then. And we assume Tifa gets Yeah!s. It helps that the first 4-5 are ridiculously easy to get. Not sure what ARPG stuff you're talking about in comparison; note that we usually DO assume that combos can be executed properly, and those can certainly take some skill for some games.