First of all, like Greg said, probably be best to get a lawyer and CPA involved. In my experience, It is usually worth the couple hundred bucks to get that advice.
I am not a lawyer or CPA, so this isn't law/accounting advice. Just my opinions. Also, I am usually on the deal/labor side of projects.
1.Fairness is in the eye of the beholder. I have usually offered my lenders my preferred terms and cash amount of the loan (and I usually do it as interest only loans, payable monthly or at the end, but P&I payments have worked as well), but with the option to negotiate. As far as timeline, I have done 6month, 1 year, 2 years, 3 years, 5 years, 5-7 anticipated , and 10 year. I think the timeframe is usually how long you will be fine having the money tied up.
2. I prefer to have my money partners just get a guaranteed rate of return. It makes documentation and understanding of the deal much clearer. If this a friends/family partnership, this is a lot less likely to make the next Thanksgiving/ tailgate awkward.
3. For me this would normally be the most difficult, stressful and potentially damaging to any relationship. I will not set this up. Everyone will have different ideas about where/when to spend any money, what the value might be, etc. For example, you might need a new HVAC unit in year 3. Since you are paying for it, who would still make the final decision? Does it go up to a vote? Does everyone try to find the best price and go with that?
One caveat. I have thought about giving an investor a lower rate of return ( say 3%), then a 40-60% return on the sale of the house. If they were to refinance, it just needs to be clear what bank/appraiser/whomever will be defining the ARV.
4. You should have the first lien on the property. For any changes in the dynamics, this needs to be in the partnership agreement. The lawyer can really help you word this. But I would be very careful, in my opinion, relationship dynamics are always changing, especially over a 5 year period.
5. Look into cost segregation/ bonus depreciation. I'm not sure if it will be beneficial in this case, but that is the first that pops to mind. I'm sure there are others.
So,
A. Set it up so you are getting a guaranteed rate of return.
B. You hold the first lien on the property.
C. Set up the terms for as long as you are comfortable not having access to the cash.
D. Give that information to the lawyer to draw up the agreement.
There are also typically other clause's, the prepayment agreement is one that pops into my mind, that the lawyer should know and understand and can guide you on.