You develop a House Rule that covers all the situations you have questions about. You might want to have a discussion with the majority of your "regular" players. Once you have done that your House Rule can be structured in such a way to resolve whatever issues you need to resolve. One thing you need to include in your discussion is the effect of your local Tax Laws and how those laws may effect any deal made.
For example, I know of one location where the players are allowed to make deals, but the "official" payout amounts for each position are used. So, for example, if a deal includes 1st and 2nd place, payouts are $2,000 and $1,000 and they decide to split the money evenly and play only for the "trophy", player 1 is at a distinct tax disadvantage as his final "earnings" are $1,500 but are reported as $2,000. Player 2 is at a distinct tax advantage as his final "earnings" are $1,500 but are reported as $1,000.
Chet