They use a percentage when maintenance/cap ex is not primarily a function of rent. 

If you have multiple people on the company, you ideally want the person with the highest credit score to have the most ownership percentages. 

If I was involved, I would want to give you fair market value for the land, and only give up a small percentage after that, because the builder will be doing the heavy lifting on this project. 

Looking for help on what percentages should be for expenses at this scale and any creative financing options?

You will want a Realtor who is looking out for your well being and not trying to sell you the most expensive property and a property management company who is honest and has a track record of high performance in rentals - both booking percentages and rates. 

Others might still go for a percentage, but since it’s off-market and might not involve as much legwork, they might be open to something less than the usual rate.Who throws out the first number?

On the owner-occupied loan - For 1031 purposes, the IRS doesn't care about the percentage down payment (although, if he wants to defer all taxes from the land, then all of the equity from the sale does need to go as down payment on the investment half of the duplex). 

Offer move-in incentives (hard dollar amount, not a percentage)4.

While there were challenges, such as adjusting to the nonrefundable deposit requirement as a percentage of the purchase price, I remained steadfast in my vision.

Just meet with an attorney who can draw up the agreement with the percentage of ownership with the terms of the agreement and then on deed, it would list both partners as owner with their percentage and it would stay that percentage for capital gains and expenses pro ratta.