BM bringing friend instead of date?

I think you are overthinking this right now. A lot can change in the next 6 months. I think you are good with extending a plus 1 to her and she can choose who to bring (be it a friend or a boyfriend). I would say that if the significant others of your bridal party are invited on the party bus then whoever she brings should be invited as well. As for getting ready that is up to you and her. Again, it should be something you think about and even ask your friend about but more in the May timeframe when she knows for sure who she is bringing.

I wouldn't stress yet but I do think it was presumptuous that she assumed she was getting a plus one if you never told her that. I have been in weddings single and not been given a plus one.

If you did give her a plus one, and she already told her friend she was it then if she gets into a relationship she has to deal with that problem - not you. And I don't think you should invite the friends husband. But if the friend comes I do think you should treat her like you're treating the other dates

If she has a plus one she can bring whoever she wants

I don't necessarily think that every adult should get a plus one (I'm not talking SO's here), but I do think that your bridal party should; it's an act of respect to them.

She doesn't have to get ready with you, her husband doesn't get an invite.

Sounds good! I was planning on giving them all plus ones for sure! It's just when she mentioned it the other night at dinner it caught me off guard because I wasn't actually thinking about it yet. So then I stared thinking about it and that usually never leads to anything good. I guess the main thing is that I wasn't sure if it was still rude to not invite her husband since technically my friend invited her. Sounds like it should be fine though.

As a general rule I give every adult a plus 1, and they get to determine who that is. I would not invite the BM's friend's husband (plus 1 does not get a plus 1, haha). I don't think it will be awkward. The friend knows what coming to this wedding will mean, and that's up to her to decide if she wants to come. Not something you personally need to stress over. Everything will be juuuuuust fine

Guests/BP can invite anyone they want if you give them a plus 1, which you should.assuming the guest is an adult, they will figure out how to entertain themselves during the day.

Instead of a head table, I suggest seating your bridal party among your guests so they can sit with their plus ones. This eliminates any awkwardness for plus one's who don't know anyone else.