Before answering your specific questions, I want to comment about healers taking vuln stacks.
It's rare that one or two vuln stacks are going to lead to death even for healers and DPS in normal content. That's part of why some players will intentionally eat mechanics. I have intentionally taken vuln stacks as a healer if it meant I could keep another party member from dying or prevent a wipe by raising a tank or the other healer when things are going badly. Vuln stacks can also mean the healer is still learning the mechanics.
Vuln stacks are not a reason to panic or disband in normal content. Once we get into EX or Savage, it's obviously a different story.
Now for your questions:
1a - Depending on intended difficulty of content, yes.
1b - Part of the community have been doing that all along. Nothing new there. It existed even when I first started healing in WoW 16 years ago. Your experience will vary depending on what players you get matched to. I doubt many will stop playing healer completely. I do expect some to set limits for what content they'll be willing to heal.
1c - No. I wouldn't recommend jumping straight into any of the expansions' content as a fresh healer. It is a learning process.
2 - Yes unless the healer isn't even trying. How I adjust depends on what job I'm playing. Some of your options do not apply because we can't change jobs in the middle of most duties (treasure map dungeons are the only ones I can think of off hand) and never while in combat. If the healer is acting like they're overwhelmed, I'll offer to give them advice if they want it.
Regarding your story time:
1 - 30 minutes is not an acceptable quit point when its the first week of the content's release and most players are going into it blind. If there are problems in the group causing wipes, work as a group to try to fix the problem. Don't just give up. The only time to quit that early is when someone within the group causing problems shows they're not interested in trying to improve and is expecting the rest of the group to give them a hard carry,
2 - Honestly depends on the individual situation. I had one party disband on the 3rd boss Tuesday morning. While I know I was part of the problem, I don't blame the party because the other healer had been constantly mouthing off about how it was getting boring (even though they were dying far more often than I was). I wasn't angry though I do wish players would have more patience on occasion. Not everyone learns at the same rate. I don't switch jobs unless part of a group that is going to requeue together. Instead, I work to identify why I'm failing and correct the problem. I did requeue and the next party did manage to down the boss on the 2nd attempt although it was just the 2 tanks left standing for the last 7%.
Most of the problems I saw when I did the raids Tuesday morning came down to the usual. It was new content and everyone was blind. Most players were going in barely above minimum ilvl which means mechanics are going to hurt more than once they get up to ilvl 710 (or higher). This is going to continue to be a problem for the next few weeks as more players complete MSQ and jump into the raid as soon as they get a couple of pieces of gear that will get them to the minimum ilvl.
Some players expect instant success and can't seem to understand why that's not always going to happen. You just have to learn to shrug them off. You're unlikely to change them and they're not going to change for the sake of others.