Not sure exactly from your questions who you represent. I assume both so apologies if I get it wrong. First, if you haven’t already, make sure to get a conflict waiver. The reason for that is one of the options to reduce a spouse’s liability is to request innocent spouse and throw the other taxpayer spouse under the proverbial bus.
Question 1: I already mentioned innocent spouse but you can also request an offer in compromise on that tax debt. Also bankruptcy may be an option for the spouse (obviously you can’t request that in CDP but it is still an option). Finally, you have penalty abatement.
Question 2: I am not so sure about the answer to this question but I’d think that you would just request that the accounts be mirrored so that each spouse could set up a separate resolution regarding the tax debt. So I don’t see a problem with separate 433-As. You can do OICs for a divorced couple on joint tax debts (assuming that this is a feasible option).
Question 3: Normally, I just file the 12153 and get cracking. It takes a couple of months to get a call to set an appointment with the settlement officer. I did, however, have had one case with an RO where the RO allowed me to prepare and file the 12153 with them (so it was timely filed) but we held off acting on it to see if we couldn’t work a deal out (and eventually we did). I don’t know if ACS can do anything like that so probably best to file timely unless you are close to the CSED then I’d see if I can’t get an extension so I can file a request for an equivalent hearing (which normally halts collections but doesn’t stop the running of the CSED).
Question 4: I think you’d need to know more. Not enough here to know what other things out there you can do.
Good luck. I wish I had more CDP hearings. Way better dealing with IRS that way than through ACS or the RO>