1)'Our dinning room is a place which we keep strictly for eating as opposed to/whereas the sitting room, which is for sitting,talking and watching TV.'
** I think that if you choose whereas the sentence is incomplete, the other reason is that :whereas is used to contrast facts, but as opposed to is used to contrast things.
2)'the weather is likely to be dry and warm.In the far north-west of Scotland, however/whereas, it will be wet and windy.'
** however here means although
I hope I am correct, anyway we will see what tdol says about that