1) Wear them for anything that doesn't involve running. Not advisable to wear them for running as the heel has no give at all. I have never heard of anyone saying to not wear them for DB work. If the DB work doesn't involve running I see no reason to not wear them. Wear Oly shoes for the OLifts, squats, any gym work that doens't require you to run.
2) Old lifters do lift. Look at the Masters, plenty of active older lifters kicking about. Just that they don't post on forums for the most part. I guess it's hard to lift 30-50% less than what you are use to lifting. Spoke to this Masters lifter at the English, forgot his name :(. Back at his prime he was a 155 207.5kg guy!!!! FS 235kg and BS 265kg. He said it was lovely to BS 250 for triples as at that weight you get a crazy amount of bar whip. I told him I'll tell him what it feels like when I get there :P. He has the Masters 45-50 WR Cj of like 171kg! Mental.
Long term affects? Depends how you are built and how hard you push it. He is like 52-53 and still kicking about and not taking it easy. Just be sensible and don't go crazy when you are injured. But if you do anything for long and hard enough it will beat on your body. If you are older you can't train like some young guys in their 20's.
No weight limits imo. Just depends how your body is built. I tend to find that bigger guys tend to have less injuries with their knees than smaller 56-77kg guys. Less fat and muscle around the joints to soak up the punishments of training.
3) back injury during DL. Not great, out for 9months in 2004/05, nearly surgery, prolapsed L4. PB attempt, should have dropped it but grinded it up, made the lift but nearly ended my lifting days.
Literally about 2 or 3 wrist injuries catching the Cn wrong. Out for 2 weeks at a time of no over head work. Hammered the squats so not too bad.
Other injuries were non lifting related:
-moutain boarding : 3months, tarded
-kite boarding : 6 weeks
-gladiators tv show injury on mcl, out for 3months
All in all I rarely ever get injured from OLifting. Everything is controlled and no outside factors. You build yourself up to lift the weight. You don't go from nothing to trying to CJ 165kg.