Hey Adam, I will address your questions with a story:
When I was flipping houses in Western Nevada in 2014 as a side gig, I brought in a friend from my normal work on one particular house. We had known each other less than a year but we seemed to hit it off, and besides--he was pretty handy and had some capital to work with, so I figured why not? I hand-wrote an agreement specifying how much money was involved, what the splits would be, who would do what work, and what would happen if everything went sideways. I copied the agreement and we each signed both copies, then each kept a copy. Everything went fine with the first house and when it closed we made hand-written notes to the agreement letting the proceeds roll to the next house, which also went well. By the third house and on we simply took care of things verbally.
Due to a subsequent reassignment, we later parted ways but we did so happily, still friends, and each a little bit better off financially. Now, I fully expect someone to reply to this post telling me how stupid that was, but it worked for me.
And that's the point.
You have to find an arrangement that works for you and your partner(s). There is no magic formula for what will work and what won't--that needs to be negotiated based on who you're working with and the details of the project. If you do not feel comfortable with a "cocktail napkin" arrangement like I had then you should probably seek legal counsel to draw up something that will withstand the scrutiny of litigation. And an accountant can advise you on the tax ramifications (in my case, we did not split things 50/50 because the houses were in my name so I paid the capital gains at the end of the year, but I did provide him a 1099).
This does not directly answer your question but I hope it helps.
Best,
Vincent