First, allow me to thank you for the attempt at meeting a middle-ground on Mercy that would aim to appeal to both Revert and Rework lovers of Mercy’s kit (and of course those in between). I’m not sure if my input adds to the discussion, but as someone who is pro-revert, I will say that I do like some of these concepts you touched on.
I like the concept of building a res charge. To add to that to that, I would make it only usable when she Ults, instead of her E move. Her res charges can have a 1.25s cast time (instead of almost 3 seconds) which lines up with her "Heroe’s never die voiceline, LOS, and a 30% damage reduction. Besides the free rez charge, the rez charges you build would all be used at once and the ones you gained could be lost the moment you died before using it (meaning, whatever resses you saved through healing are deleted once you are killed and you are back to 1 res). This would still allow for your dynamic goal-oriented play, while encouraging good ult economy. An optional idea would be to make it so each death removes a res charge instead of all your resses going away from a single kill. If this system is too punishing. 
I personally would switch Valkyrie and Res. I believe having Valk as an E move with a single stronger beam and limited flight (8 second duration on a 25 second cooldown maybe. Not sure, the timings can be tweaked) would be a great utility ability for Mercy, allowing her to escape, get to high ground, or output more healing to build more ult charge. I don’t personally think that she would need a longer beam (though the concept of mercy with a healing stream across the junker-town map sounds adorably hilarious), it may be a tiny bit overkill for her kit. The range she has currently would suffice in my opinion. With how it is now, I wouldn’t have much else to change other than it’s duration and being a single target beam heal at 70hp/s, which would be more than enough to make it an impactful E move in my opinion.
I don’t think there’s an issue with how much she damage boosts now. Personally I wouldn’t mind of people think it’s too weak and they buff it, but right now, I’d be fine with it staying at 30%.
And well, that’s pretty much my suggestions. I guess that would be my ideal “valkyrie / revert sandwich kit”. Not something I personally would choose over a Revert + tweaks per se, but definitely something I would enjoy playing if Mercy gets another rework that must incorporate Valkyrie in some shape or form. Just my opinion. Either way I’m all for earnable res and res charges as a concept. I think if anything should be added to current Mercy, it should be that. 
~Sincerely Yours xoxo,
a Lover of True, Fair, and Fun Balance.
xavvypls 