Ah, that would be the problem, my friend has our only rulebook and I just have a working knowledge of it.
Anyway, I would say that the D-Cannons go in the Fortress, assuming they can fit. Because that is where the brunt of your firing power is, so why not go whole-hog. Also, it offers extra protection for them. I would imagine the character goes with them for the same reason the exarchs fire the Icarus Lascannons, if you roll sixes to wound, I'd imagine you could allocate wounds, even if it's a blast, based on what you've already said.
Partially correct in placment but not in practice but close!
Farsser takes prime power from divination tree and guides the squad allowing the shots out of LOS to be super accurate.
1 squad goes in the main bunker with firs points the otherone is deloyed behind the fortress and fire indrectly. Since i dont have fortune leeping them from being shot in the first place is the best defense
Secondly i use them as area denial and point defense, same as the dragons. There is nothing that a d-cannon cannot maim/ kill so most things that need to get close to hurt the fortress are eradicated.
You may notice a theme, so far all my weapons either have really long range and deny cover saves, or dont need LOS and deny cover saves, also because of the Tau most everything ignores nightfighting if it comes up.
The D-cannons are excellent at area denial and you can avoid them because they are protectig the fortress, AV14 is very durable at range (especially if i can give it a cover save) so you need to come to me.
Can you tell me how the jetbikes might be used in conjunction with the above?