1) Why do healers cast Stoneskin/ Protect when there are still some people watching the cutscene? You're just going to have to cast it again in a few seconds, why not just wait and save your mana?
Because some groups do not wait for any buffs aside from Protect. And some don't even wait for that. So, as a WHM, I will individually hit people with Stoneskin as they load and Protect can wait until everyone is in. Because I find it is slightly more likely that groups (tanks especially) will wait for Protect than Stoneskin. And even then I sometimes have to Swiftcast Protect on the run.
2) Casting Medicas when only one or two people really need healing, to the point their go to reaction seems to be Medica II > Cure II. ??? It's such a waste of mana and drives me insane. It doesn't even grant enough HP for the people that you're doing it for.
Depends. Is there a big room-wide aoe coming? Then I'll start Medica II in advance so it hits when the attack does. Otherwise there is no need for it in that situation. If the two people being hit are dps, regen will usually do the trick.
3) Not resurrecting your co-healer if they die (and there's plenty of time/mana.) Why? Is it out of some sort of perverse need to try and solo it? Your fellow healer isn't your rival, they're your friend. Saw this on Shiva, they were flinging Cures at players with 95% HP almost the whole time while the other one just lay there.
Again, depends. Has the healer already died several times, either due to being new to the fight or general failings? If so, eventually I'll stop raising them. Especially if my mana is running low. Same for any dpsers actually. If the fight is not a hard dps race and I have to choose between saving my mana to keep the tank up or raising that dps who keeps hitting every aoe, that dps will be staying on the floor.
If there is a summoner in the group, I'll leave any raising to them, unless they need raising. They have mana to spare for that sort of thing and I probably won't.