I think the PCs' not adopting the plan is a bigger problem than the fact that the partnership funded the contributions. I would view the partnership's funding of the contributions, which presumably means that the partnership withheld the amount from distributions going to the PCs, as if the money had been paid to the PCs and then contributed back to the partnership. But of course, for the portion of the year that the PCs were the partners, contribution formulas and 415 would need to be applied based on the owner-employees' W-2s from the PCs, not on the draw going to the PCs, even if the PCs had properly adopted the plan.
I doubt if the PCs' failure to adopt the plan could be fixed in EPCRS, but you can call the IRS and arrange a pre-submission conference and ask them.
If they were individual partners for a portion of 2021, you might be able to work with that.