Sorry Barrister had to dash -- I'm afraid "life happens" to us all!
I'll try and provide some perspective. First, be advised I am licensed in California, no Michigan. Also be advised that lawyers here cannot give specific legal advice here, rather only address legal issues generally.
I'm sorry to say, that I don't think there is a simple answer for you.
In a complex ownership-investment situation as set forth in this scenario -- with various ownership proportions, debt service proportions, property improvement investing proportions -- the first thing to be sure of is that your percentages add up.
People are free to contract, and free to agree to pay more for apparently less, etc., becasue people value different things differently. So long as there is legal consideration. a contract is enforceable.
The issues to consider and address in this scenario:
-- whether the mortgage dictates restrictions on form of the property ownership or transfer, or if it can be renegotiated;
-- property ownership proportions and form (tenants in common, joint tenancy, trust), and which needs to take into consideration the goal with respect to ultimate ownership/disposition of the property upon the death of current proportional owners and investors.
-- investment proportions, both in the form of mortgage payment contributions and property improvement and maintenance, among others;
-- division of return on investment, whether monetary (from rents) or in-kind (use of the house in some measure) or potential ownership interests.
So taking these issues into consideration, a group can draft a contract addressing these aspects.
Another route would be setting up a limited partnership or a limited liability company or other corporate form.
An alternative would be to create a new trust to own and manage the property with the various family member being the beneficiaries, with dividend being money, use or future ownship interests.
A trust may be the simplest and most flexible, expecially when it come to ultimate disposition. But if a family member is the trustees managing the property, there may be in-fighting & hard feelings if the other family-beneficiaries dislike trustee's decisions on property management and investment.
Of course, this is a pitfall in family parterships or corporations as well.
I hope this helps with some perspective, though I know it was not the "simple answer" you were probably hoping for.