"This client will likely hold onto all shares for some time, making the paperwork trail / history even more perplexing. Shares already in place are from ESPP and ISO plans. "

I would argue the opposite -- the longer they are held, the easier it will be. Both ESPP and ISO are statutory stock options, meaning they are eligible for cap gains treatment instead of ordinary compensation, if they are held for certain minimum periods from grant and exercise dates. If held for the statutory minimum periods, ISOs never trigger wage income, and ESPPs may or may not (only if there is a "look back" discount available to the beginning of the grant period). Therefore your basis calculations will be easier.

So far, neither a Form 3921 nor 3922 has been produced."

In most cases I've encountered, you can get the equivalent information from the employer and/or the brokerage. After all, no one asks the employee to make exercise decisions without all the relevant information at hand. The 3921/22 is really only to keep the IRS in the loop, it's not meant to be the only source of info.