(TL;DR at the bottom because I rambled a bit--sorry!)
With the first case, I agree with you on everything (something that you only have access to upon death isn't such a bad thing when your role requires that you put your life on the line--Aatrox is an alright example, but someone like Sion is probably the best implementation of it to date due to the fact that he wants to get as deep into the enemy team as possible and has extremely limited means of escaping), except for Zyra and champions like Zyra fitting in that caregory, since positioning and timing are elements that are core to the gameplay that surrounds them. I assume there's a reason champions like Veigar, Annie, Fiddlesticks, Karma, Malzahar, Ryze, and Cassiopeia (who probably has the worst range of them all)--who all fit the bill of being squishy, immobile, mid-ranged mages with windows of power limited by ability cooldowns--don't do something odd when somebody on the enemy team outpositions them and turns their screen grey. I think for Zyra in particular, something that rewards her for staying out of harm's way or gives her a tool to deal with champs like Fizz who can jump straight to her from 1250 units away (and champion LoS is 1100 units, if I recall correctly) would be more appropriate, but I think this actually brings us to the second point.
With the second case, I'm going to say that it's not that great a justification because you guys have come up with much more elegant and creative fallback patterns for many other champs, and have left other champions to suffer from that feast-or-famine gameplay (Yi is probably the most tragic example) for over half a decade. Yasuo, for example, doesn't Seppuku himself through an enemy champion Yoshimitsu-style when he dies, he creates a projectile blocking wall that his entire team benefits from. Akali doesn't burst into a cloud of kunai when she dies, but she can create a slowing field. Even Kog'Maw has a much longer, more potent slowing field, an armor/MR shred, and his ult's reveal to contribute to his team when he isn't doing too hot.
It just feels kind of bogus, having a chunk of your ability kit denied from you when you're actually playing well (Sion's expected to still use his because "playing well" for him could mean eating 3 ults that his carry no longer has to worry about), when other more satisfying solutions could have been applied. Allocating power in ways that give enemies some way to respond when certain champions are "crushing" in any particular game, I think, would ultimately be a more satisfying way to prevent that feast-or-famine.
For example, toning down some of the raw power in Kog'Maw's kit and tying it to an Innate that increases his damage dealt to an individual target after attacking them a set amount of times (tying in with both his acidic theming and his role as a marksman) makes him a bit less of a nightmare, because now if he's doing really well, circulating your frontline before he reaches that peak or keeping fights limited to short skirmishes are viable ways to undermine his power--I believe this is actually something that was done with Ashe, who could be 13/0/5 and still be dealt with if measures are taken to mitigate her strengths.
For Zyra, something along the lines of taking some immediate power away from her plants (since right now, they're basically DoTs that are occasionally applied at the end of her abilities and "drop two seeds before throwing a Q or E" doesn't create much different gameplay than Brand going for a pillar -> stun combo or Vel'Koz guaranteeing a few Innate procs by setting up with his weird knockup) and pushes her towards a more unique style of play might be worth exploring. Perhaps instead of going, "hey, Zyra's a champion, and all champions should be designed around contributing to huge teamfights," turning Zyra into a champion that broadens the range of strategy available to players by defining her strengths around cultivating her plants and tactically spreading them around the map could make her a champion that's unique in her own right (beyond having lamer versions of Heimer turrets) with counterplay that's unique to her (kill the plants before they grow to deny her map presence and damage/utility), and ultimately make her a lot less feast-or-famine without locking a fifth of her kit behind one of the intentionally worst feelings in the game.
TL;DR -
1- Post-death abilities are cool on champs whose gameplay puts them at direct risk of dying; post-death abilities on champs whose abilities/roles don't require they throw themselves into the enemy team don't really make sense, and I assume Veigar/Cass/Ryze/Annie don't explode upon dying for deliberate reasons. I didn't mention this in my spiel, but Karthus and Anivia I can mostly understand from thematic viewpoints, but I think those qualities to them could be improved as well.
2- I can see your train of thought on this one, and you're by no means incorrect, but there are definitely more creative solutions to the feast/famine problem that don't involve locking away 20% of any given kit. Either that, or I expect Yi/Vayne/Draven/etc. to explode on death in future iterations of their gameplay kits. I hope A) that's not the case, and B) that we eventually get better solutions to Kog'Maw's and Zyra's Innates.
Thanks for reading! While I don't necessarily agree with all of your points, I'm glad you guys are open with us and don't hesitate to explain the justifications behind your decisions.
and 
Zyra transferred her consciousness into a human body for a second chance at life.
...
Potent magic surged through her vines, and she devised an elegant but risky plan to survive. Using the woman’s memories, Zyra poured her newfound magic into the creation of a human-shaped vessel. She didn’t know what sort of world awaited her, but she had nothing left to lose.
...
She was reborn, and there was so much now within her grasp.
