I took some time this morning to check the Console for any OpenRCT notifications.
When I try to open a park from another install of RCT (Open or Classic), the OpenRCT2 process throws this error: RECEIVED OUT-OF-SEQUENCE NOTIFICATION: 406 vs 537, 513, <private>
(The 406, 513 number vary. I also saw 408 vs 539, 410 vs 541, 413 vs 544, and 415 vs 546. All else remains constant.)
I also saw this from systemsd (Symptom Evaluator): "Received CPU usage trigger: OpenRCT2[7167] () used 90.00s of CPU over 127.82 seconds (averaging 70%), violating a CPU limit of 90.00s over 180 seconds."
and from the kernel: "process OpenRCT2[7167] thread 1360549 caught burning CPU! It used more than 50% CPU over 180 seconds (actual recent usage: 70% over ~128 seconds). Thread lifetime cpu usage 90.056228s, (87.413305 user, 2.642923 sys) ledger balance: 90004389428 mabs credit: 90006950168 mabs debit: 2560740 mabs limit: 90000000000 mabs period: 180000000000 ns last refill: 127817334688 ns."
I did a bit of hunting about the Out-of-Sequence notification thing and it looks like a very slight error in the code can cause it ( a : where there shouldn't be one, or no : where there should be). Previous versions of OSX ignored these, but Mojave is stricter.
No idea about the CPU usage things.
Should I lay hands on another computer or have half a day to install linux on a spare drive or something, I'll test to see how things work there. I expect all will be fine and these are just Mojave being Mojave.
EDIT: I also tried granting OpenRCT2 Accessibility rights and Full Disk Access, hoping the file opening issue was permissions related. Alas. No joy.
Thanks so much for all your help and thoughts!