1) It does not.

2) To be clear, I am in favour of making assumptions that allow the spell to function in the way as described, but also to be clear, that's an assumption which is not supported by any rules text and nothing suggests multiple casters working together is the intended solution in the text. You'll need to talk that over with your DM.

3) I mean, it's on the list of things that the DM will need to fix if you want the spell to be able to function as described, yes. Particularly since some summoning spells even have a casting time longer than 1 action.

4) Again, talk to your DM. The spell is clearly described as being usable in a certain way, yet the mechanics do not support that use of the spell. There's obviously a mismatch of some kind here, and you'll need to talk to your DM to sort it out basically no matter how you look at it.