If someone throws in 2 x 1000 is there anything wrong in the dealer confirming the action in the case of 200/400 raise to 1200 if there is or can be a doubt?
No. In fact that's probably the sensible thing to do. If someone were to object though then I imagine you'd have to rule it as a call.
So your saying if he throws in 4 x 500 chips its a call as 3 chips is less than 1650
but if he puts in 20 x 100 chips you would make it a raise
not a good rule in my opinion
it is up to a player to protect himself during a hand if he puts in multiple chips and more than 50% on top of the bet then it should be a minraise imo