1. Rogue for Fane is a viable choice in my opinion, it did it on my vanilla PT on tactian. You can set up incredible turns with proper use of play dead & timestop, especially in conjunction with adrenalin rush.
However, it might be a good idea to include an outside source for poison in order to heal him, especially if he gets CCed. Ingenoius got changed as well, so it should be even better.
2. A summoner - in general - is very flexible. The question is, at least in your suggested group, how do you want to create easy elemental infusions with surfaces?
Furthermore, your setup lacks magical damage. This may come down to preference, but I am not a big fan of pure damage type groups. Imo it´s much easier to mass CC with a proper mixed setup. If you aim for tactitian, some fights will be much harder with the lack of one damage type.
3. Ifans crosbow sure is great when you aquire it, but not mandatory. Flesh sacrifice will be useful during the whole game, the crosbow just gives you a slight powerspike around lvl 5 - 7. Break the shackles is also a thing.
4. I just provide my current setup, which is fun to play and quite strong. Maybe you find something that suits you
Ifan: Scoundrel/Polymorph and 1 point in Geomancy for Venom Blades/Stoneskin. (should work with Fane as well, I just picked Ifan for his story arc)
The Red Prince: Warfare (2-handed), 1 Geo for Stoneskin
Lohse: Geomancy/Pyromancy with a crit build
Sebille: Summoner/Hunter with 2 points in Hydro
