I'm on my first play though and I tried to avoid all hints/cheats/spoilers etc.

I aligned myself with all 4 factions and everything was pretty much under control until now. I have 2 conflicting quests (Mass Fusion and Tactical Thinking). Because Tactical Thinking is active I can't "Inform the BOS" for Mass Fusion.

I read somewhere that I could avoid Kells and talk to Ingram, but all she does is tell me that Kells is waiting for me.

I proceeded down Tactical Thinking, hoping I could fix the conflict, but I guess not. I didn't like how that mission ended.

If I back up before Blind Betrayal, will I end up at the same decision in another couple missions anyway? IE: It is already too late to get the 3 Ally Ending?

It is going to svck if I need to attack the BOS after I rebuilt Liberty Prime...

Talking to Kells starting Tactical Thinking would lead to the point of no return where you become the enemies with the RR, Mass Fusion is the point of no return and would turn the BoS hostile. I personally go with the BoS ending, but if you want to maintain the RR in the BoS ending through an exploit then inform the BoS of the Mass Fusion then talk to Ingram, it will skip over to Spoil of War and as long as you don't talk to Kells, you don't have to start Tactical Thinking even after the game has ended.

I believe the problem is that you can't talk to Ingram until you talk to Kells because Tactical Thinking starts the instant you finish Blind Betrayal. She will simply direct you to talk to Kells. PAM goes on lockdown, but you're not officially enemies with the RR until you talk to Kells. It was possible previously to talk to Ingram, but that got patched, from what I understand.