Why do you think we would miss out on Dembele because of Deulofeu? There are 3 possibilites with Deu if we bring him back imo.
1. He shows he's ready for Barça, establish himself as our starter (most likely at RW) and stays here for years to come. In this scenario we probably won't need Dembele (as much as I like him, if we can get great production out of Deulofeu for 12M, that's a lot better than spend 60M+ on Dembele) and could maybe use "Dembele money" to buy someone like Dybala in 2-3 years as longterm Suarez replacement.
2. Deulofeu fails to make an impact here and we sell him in a year or two. Keep in mind that his net transfer fee would only be 6M (our buy clause 12M minus 6M we received from Everton in 2015) and we can probably sell him for a fee at least as high next summer. And then we can buy Dembele or maybe another young player that will emerge next season if Dembele won't progress as expected.
3. He won't be good enough to become a starter but he'll show enough to be a good backup for MSN (he could probably cover all three positions upfront) for 2-3 seasons, maybe even more if he would be happy in that role. If not, we can sell him for 10-15M or even more. I think that scenario is the most likely to happen.
I really don't see any downside in bringing Gerard back and see what he has to offer. We could spend those 12M much worse than on him.