This seems right to me, play Siggy next to Gana OR Schneiderlin, since we see how poorly the AND works out. Think he might complement Gana rather nicely.
I don't think dropping Sigurdsson deeper is going to do anything. We need someone who gets on the ball a lot and passes it forward. From what I've seen for club and country, Sigurdsson isn't aggressive enough at seeking the ball and his quality as a passer is questionable at best. We need an entirely different sort of player in that area. We can't expect him to do something he's never done before.
At some point, Silva needs to consider Dowell as a starter, just to see what he can offer at this level. He showed some serious promise as an open play passer in the Championship last season and he's pretty good from dead ball situations, too.
On the topic of Calvert-Lewin... he should have had the second half of last season to play regularly and develop since we had absolutely nothing to play for. We never should have bothered signing Tosun.
Since we didn't do that then, I think we need to have a serious look now. He's clearly not a winger. Play him in his natural position and see what he can do, especially with Richarlison and Walcott flanking him.