You can have two slot guys.
That would be with 4 WRs on the field. In 3 wide, the side with the single receiver could have a tight split, but that's normally the side where the TE is aligned so that WR is technically an outside receiver.
I actually think if they wanted to do that or need a backup for Beasley, they would use Dunbar as a WR. Garrett talked about Dunbar being such a good receiver that he could move to WR if they needed him there.
Beasley is already the primary reason that Escobar does not get more snaps. If not for Beasley, they would likely use Escobar more as a 2nd receiving TE. He is reported to be lining up split out wide a lot in training camp. He might have the best hands on the team.
I like Whitehead, but if he is not the returner it's highly doubtful that he would be active. For him to make the roster without being the returner, he'll have to be a fantastic WR. As strictly a WR, if it's close between Whitehead and a big WR, the Cowboys seem to prefer keeping the big guys.
I think he will be the returner so the other issues would be a moot point. He has also been practicing as coverage guy on special teams which would really help his cause to make the team. If he is only the returner, then they might prefer to just have Beasley be the PR and use the extra spot on the 46 for an ST coverage guy. KR is really not a big deal anymore because kicks most often just result in touch-backs now anyway. PR is the primary concern to me.