Further exploration seems to be pointing to the specific issue being specific custom scenery objects.
I loaded up a (really) old saved game (from the super early days of custom objects) with only a handful of custom objects on both my old computer (Macos10.11.6 running openrct2 0.2.2) and new one. Attempting to open the file on both computers brought up the "missing or corrupt" object file dialogue. I selected "download all" on both. This was successful on my older computer, with the saved game being openable after closing the error dialogue and re-opening the saved game.
However, on my newer computer, I get stuck on a loop with the program being stuck on certain objects. Digging through OpenRCT2 application support folder, these objects are being downloaded when I select "download all" noted by the multiple copies present in the folder. It seems that maybe OpenRCT2 is forgetting it just download them, or doesn't realize the download was successful?
It looks like custom objects from standard benches all work fine, but my customization of said benches means there are a handful of objects that confuse the program. I'm currently working up a list of specific objects that are causing consternation.