Your question a bit confuse.

Majority DLCs are region locked, but not all, some DLCs are region free (very rare). So basically, R1 DLC work with R1 game only. R1 DLC won't work with other region game (like R2, R3 or R4)

If your game is R1 and you purchase R1 DLC, you can play the game with those DLC purchased from R1 account through different region account (like R2 or R3). Please take note, this still not 100% correct, some DLCs (like GOW Ascension) is account locked, you can access the DLC via account which redeemed/purchased it only. Although game and DLC are from same region, you still can't access the DLC with different account if the DLC is account locked.

Yes you can.
Majority of games from any of the PSN stores will work across your PS3 as long as the account you bought/downloaded from remains on the system.

Do note there are some games — notably, Persona 4 Arena — that is entirely region locked. So if you bought it on the US PSN store, it won't work on your PS3 which I assume is a MY/SG set.

Yes bro.