So I had this same problem, my friend and I found a work-around.What you want to do is,
A: have a friend that has live and halo 5 (obviously)
B: Go into forge and open the map that you can no longer edit due to not having respawn points.
C: Now have your friend join your game session.
D: They should spawn in as a monitor, as per the norm, and be able to then place spawn/respawn places for you.
E: Once they have placed what you’re missing, your monitor should spawn in.
F: SAVE IT !!!
I was thoroughly perturbed when I couldn’t go back and edit my map and was very happy that this worked.
HAPPY FORGING!!!
-MegaloYeti
Seeing as how a lot of people had this happen to them, I figured making a topic instead of replying to everyone was the way to go. 
MSG me on XboxLive if you want to mess around in forge or just halo in general. (I’m EST time zone, -5gmt fyi)