The data that was mined doesn't matter. Every Pokemon has placeholder moves right now and I doubt very much they will change the Eevee pattern for Gen II.
Sorry but I don't buy it. I think the datamining is more reliable than a pattern that isn't there. There is no cross-generational Eeveelution pattern to draw a conclusion from, there is only the mono-generational pattern. It is not reasonable to discard data which you have for the sake of a pattern that you would like to see happen.
My post is almost entirely about Exeggutor's C/P moveset, so it should go without saying that using Umbreon is best for that situation. But anyway, Umbreon still outlives Scizor against Seed Bomb :D. Let's check it out: Lv. 30 Exeggutor (2500 CP) with Perfect IVs. A Lv. 12 Umbreon with Perfect IVs (703 CP) dies in 17.1 seconds to C/Seed Bomd, and a Lv. 15.5 Scizor (1240 CP) with Perfect IVs dies in 17.1 seconds as well. Scizor is very frail, even with a .64x resistance to Seed Bomb.
Firstly, livetime is a deceptive metric. It may appear to, on its face, be helpful but Umbreon's higher livetime is almost exclusively due to a higher HP pool, only having 2 higher Defense than Scizor. Your Umbreon might live longer, but it will eat more and bigger potions than your Scizor.
Secondarily, this is the only test you've done. There aren't any DPS tests, no D/HP tests, no killtime tests. This is reasonable since they're impossible without moveset data... But you can't really draw conclusions without that.
Bubble strat works on this premise, but many times you aren't prestiging against low level defenders. When you are prestiging against higher level defenders, high attack will let you clear faster potentially, but mistakes (or connection errors) are far more punishing if your prestiger is frail. It is in fact faster to make it through more slowly with a bulky prestiger than to mess up and fail and have to start over with a glass cannon.
"Potentially".
I'm not particularly clear on how precisely Attack works, but assuming it's a simple multiplier on the move's base damage:
Scizor's attack stat on its own will make fights take 66% of the time assuming identical DPS movesets.
Assuming you've got Bug and Dark move double sets on both pokemon with identical DPS, Exeg's double weakness to Bug versus single weakness to Dark will create an additional discrepancy of roughly 25%. Into the number above, that makes 50%.
So yeah, Umbreon will live longer. But it takes twice as many of your potions and twice as much of your time.