during holidays, should i invite W to spend some days with Ds?

Personally, I'd let W initiate that request if that's her wish to do so.

what if she says she has plans with OM?

"What if"? You can expect that reason to surface now and then.

should i go for the pain?

No, not really.

if we spend time and we just act like friends, would it ruin the supportive friend phase?

Not unless you let it.

should i wait for her to make the move or hint something?

A big yes.