Partnership structure for providing personal guarantee

Because the SBA PG is "joint and several" it is best you structure the deal with a single guarantor. Equity split is an entirely different issue. You could look at risk allocation. For example, if you sign the PG but have no significant net worth while the investor contributes a significant amount of equity, then the investor has more at risk than you have. On the other hand, if you take the PG risk and have personal assets covering a significant portion of the loan amount, it will be a different story. It also depends on deal attractiveness and who found the deal. So, there is no "right" or "fair" answer to this question.

You could give additional equity for providing the PG, though i defer to a tax lawyer as to whether this would be a capital interest equal to the value of the guarantee or something else). If they cash out the additional equity then they probably should stay on the hook for the PG since they've been provided a cash value in exchange as a result of the buyout. I've also seen the company pay a fee % to the exited owner in exchange for maintaining the PG after an exit,