My first suggestion would actually be: Step away from the forum xD I would guess you gonna find a lot more... "extreme" people in here than in the game itself - dont let that stress you to much!
Now to go over your stuff one by one...

1) No, it is not bad to be flexible. As long as you generally know your job and perform well, everyone can keep their mouth shut. By "perform well", I mean: dont be an ice mage or a bard without roulette etc. But you do sound as if you have a good understanding of your job, you just need a little bit more confidence in that I know that I used to be the same 2 years ago (while even maining a tank...) but today I shrug that stuff off because I know that I performed to the best of my abilities and had a very good reason for the choices I made. You're worrying a little to much here - those stuff people are complaining about is mostly "Here is a person on level 60, not understanding their duties"; its not "Here is a person, doing mechanics and sacrficing a bit of dps because of that". (IF they however are complaining about that... pity them... they deserve that for being someone who needs to point out stuff like that in a dungeon... often enough those people arent that great themself but need to feel that little bit of "power")
One general advice I would have here: Try to be nice! If I havent played a job in a while, I'll let my party know - normally with something like "Hey, I havent touched this job for a few months, so I might be a bit rusty, but gonna give it my best!"

2) You shouldnt not expect that at all - if they get kicked because of that: Please, report it! I would lie if I told you that I'm "happy" to see someone who is undergeared in a dungeon, but we've all been new at somepoint! Again, I would try to be "vocal" about it, if they have bonus tell your party that thats some consolation etc. Generally: It shouldnt be to bad. (However: I would advice them to upgrade the more important parts, as in the weapon for dps for example first. Seeing a dps with a low level weapon but almost full eso-jewls is always a bit "mmh..." Farming esos in the older dungeons first, before going into the new ones while barely hitting the ilvl could be a good idea too^^)

3) Make it fun! If they got frustrated by those mechanics, put yourself into an atmosphere thats more relaxed and where it is clear that you're all there to have fun! Make that the main goal first.
Rules for that: Bring no one who is in a bad mood or has a bad attitude and only wants a fast kill. If you can: get everyone together on a voice chat, so you can laugh about silly stuff that happens.
I would also recommend to begin with Coil - you can try it synched first but if you dont make it, go in unsyched again, to get that scusses (I know, thats cheating a bit)... Show them that those raids can be fun and are nothing they have to be afraid of!

And advice I have for you btw: Get rid of those elitist friends - parsing a friend for Thordan EX? Telling him, when hes new not to fuck up? Man, thats aweful and really sucks... I was actually checking on which server you were because I would have loved to offer you some help with that kind of stuff. Sorry to say it like this, but those kind of toxic, elitist people seem to have ruined the fun (along with your self-confidence) quite a bit...

4) I dont feel like everything gets blamed on the playerbase here. Yet there are some things the devs had to do because of the way people are playing/enjoying this game. Content is to "hard"? Bäm, have a nerf! Sometimes the devs think of really cool, difficult stuff outside of the EX-primals and savage modes. But the average casual player cant handle that or dont want to put the effort into it and is rather whining about how everything is impossible to clear... so... because of that stuff gets easier and easier and thats hurting everyone who would love to have a challenge. People are also burning through content just after the new patch came out - since that content has to last for a while though, it gets grindey and even more grindey, to keep everyone playing and busy with the limited ressources they have. Specially here that grind-fest often gets blamed on the devs though! Their answer "we dont have enough staff" might be true but its hard to understand how they cant hire more people if they're making a fortune with this game (...and having the time to put items into the mog-station for a price thats making me sick)

Honestly: You seem to have run into some very, very bad people - dont let that kill your fun! Be confident in yourself! Grab your SMN and try Stone, Sky, Sea for some of the EX-Primals, to see if you have the dps to beat them! (note: that doesnt mean that you will beat them) If you can do that: great, why worry about something simple and easy like a dungeon?! If you cant: dont worry, you can improve, so go for it!
Stay away from people that make this game un-fun to you, by leaving you out and insulting you - look for like minded people, who prefer fun about BiS and who care a little more about achieving something like a Thordan EX kill together than about killing him...

Best of luck and have an awesome time in Eorzea! ;D