Thanks, Roncerr. Your thoughts led me to some workaround ideas.
I can uncheck "Include this account when receiving mail or synchronizing," on the one account that I care about most. I discovered that I can still sync this account when desired by using the inverted delta located next to the sync button on the toolbar, and selecting that account for sync. It would be easier if F5 would download all accounts, including unchecked accounts, but it performs the same function as the sync button. Any account that is unchecked for "Include this account when receiving mail or synchronizing," won't sync with the F5 key or the sync button.
Another solution would be to leave all accounts checked for "Include this account when receiving mail or synchronizing" and close WLM after each use. Until now I was only minimizing WLM, which allowed immediate viewing instead of a 3 second lag to re-open. With this solution, I can put back the checkmark next to "Send and receive messages at startup." In my case, I have MailWasher to notify me of new mail and wash the mail before opening WLM. With WLM closed, I won't have to worry about missing mail when I'm mobile due to a WLM download from my desktop.
Hopefully, there will be an update fix for the WLM bug at some point.