As complicated as this is, it is relatively simple, you could handle via a contract or an entity with an operating agreement. Contract is probably cleaner as you could more clearly define defaults and remedies in the agreement that would allow for a breakup. The partnership is easier to capture items like capital calls and change in equity. The way the money flows is pretty simple, all the equity invested is returned first - this could be down payments, interest payments, construction costs of overages, etc. Then any preferred return if part of the agreement ( you mention interest on investment). Then profits are split according to the agreement.
I have never understood why investors partner with builders or GC's, unless the GC is bringing money or signing on the loan. A profit split will almost always cost more than just paying the GC fee, and I would never pay a GC fee and split profit, unless they brought significant value to the deal- financing, cash, land, the deal, etc.
Ultimately you need an attorney involved.