When a JV partner has the seller and I have the buyer, who actually controls the deal

The probate example you gave is the one that actually matters. Six months of relationship-building on a seller who will not pick up for a stranger is real scarcity, and I have watched people give that away for a 50/50 split because they did not want the awkward conversation before the contract was signed.

Setting the split after you map the fragility makes more sense to me, but nobody does it because by that point one partner already feels entitled to half and the negotiation gets personal fast. Do you at least put a clause in writing about what happens if the buyer you brought ghosts before closing?

The assumption doing the most work in your post is that replaceability is binary, when it is actually a function of time and deal velocity. A cold-list seller who signed on the right day is technically replaceable, but if the contract is already executed and the inspection period is running, the cost of replacement is now a blown deal, not just a lower split. The window in which replaceability actually matters is narrower than the framing suggests, and most JV disputes happen after that window closes, which is why the negotiation feels backwards. The scarce role at day one of a lead is often a different role than the scarce role at day seven with a signed purchase agreement in hand.

On the sequencing question: setting the split before mapping role fragility is how people end up in a 50/50 on a deal where one partner did six months of relationship work and the other sent a text to a buyer who already passed on the zip code twice. The structure I find defensible is a base split agreed before the deal, with a written addendum that adjusts by defined contribution: who holds the executed contract, who has a signed fee agreement with the buyer, whether the seller-side partner has a non-circumvention in place that would survive the JV partner going direct. Those three documents are the actual map of which side can fall apart. The split should follow the document that is hardest to replicate without the person holding it. So the question I would ask before any conversation about percentages is which party's name is on the purchase contract and which party has something in writing with the end buyer, because whoever holds both is not actually in a JV, they are doing a courtesy split.

The split should get set the moment both partners can answer one question: what is the replacement cost of each role, measured in time, not money.

Split before is just habit dressed up as process.

The checklist a careful operator runs before the split conversation goes in this order: first, can the seller be re-approached by anyone in your network without the partner present, and has that actually been tested or just assumed? Second, is the buyer relationship documented anywhere, meaning a signed LOI on a prior deal, a text thread with ARV discussions, anything that shows the buyer trusts a specific person's judgment rather than just responding to price? Third, what happens to each side if the deal takes four more weeks, because a probate seller with a motivated attorney has a different patience threshold than a landlord who called a bandit sign and has six other cash offers coming? Fourth, does either partner have a non-circumvention agreement with their principal, and if so, does it bind the other partner or just the person who signed it? Fifth, who absorbs the EMD if the timeline slips and the deal falls out of contract, because that question surfaces which partner the transaction actually depends on faster than any conversation about splits does.

The thing your framework handles well is scarcity, but it leaves out substitutability cost. A buyer-side partner who is replaceable in theory but whose replacement would take three weeks and a price reduction is not the same as one who can be swapped in a phone call. The split should reflect the cost of losing each side, not just the probability of losing each side. That means the negotiation belongs after both partners have answered this: if you called your principal tomorrow morning and said the other partner is out, what does your principal actually do? If the answer is "they wait for me to find someone else," the relationship is load-bearing. If the answer is "they probably call the other wholesaler they already know," the relationship is ambient. Which one of those describes your buyer right now?