Everyone has made the case for the other points, so I just want to focus on question 2.
Yes. It's not irreversible, but you can make PvE nigh impossible through two means. 1. Accuracy, and 2. Endurance. Especially on a tanker that has, comparatively, low damage, and a metric ass-load of toggles. Not slotting stamina will have you chuffing blue insps like mitzi's at a rave. Not hitting with your attacks means you have to use more of them, thus you run out of endurance faster.
When you break into the mid levels (25+) you'll start running into enemies with high defences, or enemies with to-hit debuffs. You'll get pretty sick of whiffed attacks pretty quickly. Especially when debuffs are stacked. 6% chance to hit the thing that's stopping you from hitting the other thing that's kicking the shit out of you is a pretty bad time.
Maybe too literal, but yeah...you can end up with a character that's no fun because they can't do anything, or has to have a little nap and a sandwich after every fight.
As far as I'm aware, you can't accidentally ruin a character permanently.
Unless you give up and roll something else, leaving it borked and abandoned.
I will second what Six-Six said; you will get more comments about your costume than your build. I mean, people willingly take lvl 1 toons into lvl 50 +4/x8 (the hardest diff). No one, no matter how cracked they are at the game, has a build at lvl 1.
The best players have the sickest fit you've ever seen.