So, I got a message from VSL confirming that they're aware of the Logic Pro 10.5 crash glitch. They didn't say when it would be sorted out, but apparently they're working on it now.
In the meantime, I did find a workaround because downgrading to previous versions of Logic wasn't possible for me (doing so deleted Melodyne ARA data). The trick is: I NEVER USE THE KEYBOARD FOR LOGIC COMMANDS WHILE THE MIR WINDOW IS SELECTED. I found that if I select the Logic Pro X window BEFORE USING KEY COMMANDS IN LOGIC, like for example, hitting the space bar to play the song, then there's no crash. I can then RESELECT the MIR window and do anything I want within it while the song plays. Then, BEFORE STOPPING playback, I RESELECT THE LOGIC PRO X WINDOW or else the application crashes. It's a pain, and I've made a misstep here and there, but I've found it workable in the short term. Can't wait for VSL to get this sorted, but hope this is helpful to people in the meantime.