So I wanna finish the game with the Minutemen. While doing the Battle for Bunker Hill I decided to warn the Railroad since I'm gonna blow the Institue up anyway. Desdamona wasn't happy that I used the Minutemen to build the teleporter and asked me if I was gonna work for them or the Minutemen. I wanted to do a few more quests so I told her I would work for the Railroad hoping I could still get the Minutemen for the final assault. Can I still get the Minutemen ending or do I gotta reload before Bunker Hill?

You can get the Minutemen ending at ANY time. They're the fall back ending since you could become enemies with all three factions. You can NEVER be enemies with the Minutemen, because they want you to finish the game. At any point, the Minutemen can only be obtained in 1 way only and that is to piss off the Institute. You can't start the Minutemen route unless you do it. It doesn't matter what you do for the RR or the BoS at all since Minutemen quests are completely separated from other faction quests. You can do the quests for all factions until Mass Fusion/Spoils of War, then offend the Institute by killing a named member and you could start the MM ending route while doing all the quests for all the factions.

The Minutemen is the only faction where you can keep 3 factions alive at the same time. The other factions all require you to kill every other faction except the Minutemen since they are not a threat.

who the heck wants to work with Pressedham Gravy anyway?

Yo-Yo Ma.