Yes!. Discs can wear. It's by no means a common thing, but .................... The bugger is really fine dust - If it gets between the bobbins and the alloy carrier it's like a grind stone. Ironically teh worst thing you can do is try to lubricate them - the lubricant attracts more dust.
A 955 is much much lighter.
Heat: It's more a question of dissipation than generation. A thinner disc means less heat sinking capability. ESPECIALLY if they are stainless which is s cruddy thermal conductor.
This sort of diagnoses without seeing first hand is a tad difficult. Semi floating discs should have pretty much NO free play when cold. They will loosen a bit if they are HOT. If they rattle - they're dead. It depends how fussy you are - where in between you feel safe. And they are brakes. I know what I'd do. If Fully Floating discs rattled I'd bin them.
Head bearing - if in doubt - RTFM and check them anyway. Down load the
@idk free copy - grab a beer and sit down for a long read.