The corporation needs all the shareholders to agree in writing; often it's covered in the purchase agreement. Nothing goes with the tax returns, though. If they haven't yet agreed in writing, "We hereby agree to split income for ABC Inc using the specific accounting method specified in IRC Sec. 1377(a)(2)) for the tax year ending 12/31/2019" is sufficient. They need not all sign on the same piece of paper.

If you can override the K-1s to be correct in one Lacerte file, the return can be e-filed. Some stuff, like disposition of Sec 179 assets and QBI, is difficult or impossible to override to correct, so you need to stick together the return out of pages from multiple Lacerte files. In that case, paper filing it is!

When you're doing the cutoff, watch out for weird timing things that properly fall in one period or another, like fully refundable deposits received by a cash-basis taxpayer for work to be performed in the future. (Taxable to the period of receipt, alas!) You have more than typical liability for this return. If you prepare any of the shareholders' personal returns, you should also get conflict of interest waivers from everyone involved, because figuring out who gets how much income creates an actual (not just potential) conflict of interest.