Installment sale is elective. Failure to satisfy a payment agreement is common. The client tells you what to do, after you have offered your expert opinion to them. 

Is your struggle solved by just doing a bulk sale and including IRS 8594? or is the struggle in convincing the client to keep the s-corp.

I would create a copy of the return and then doing an outright sale. Let the client see how much the reporting of the whole sale will impact the client. Advise the client of the tax treatment if the agreement later fails.

Best of luck!