That sounds like an issue with Misstortion2, not with vst3shell. Does Misstortion2 work in any other VST host, both loaded directly or through vst3shell?

I can confirm Misstortion2 2.3.0 (the version I'm trying to get into OpenMPT) works on my desktop with FL Studio.
I haven't tried loading Misstortion2 with vst3shell on another program because I don't have any other programs or know of any other programs that would require vst3shell to run vst3 plugins.

You can still try using vst3shell with FL Studio or Renoise or some other VST2-compatible host to see if it works through vst3shell in that host, even if they also support VST3 plugins. That way I was able to confirm that Misstortion2 works for me both through vst3shell in Renoise and OpenMPT, as well as natively as a VST3 plugin in Renoise.

I installed vst3shell on my desktop and followed the same steps I did on my laptop with OpenMPT, and was able to get Misstortion2 working just fine with vst3shell on my desktop.
I don't have or know of any other software that I could try testing vst3shell on my laptop, the only software I have a license to is ACID Pro 5.0, and FL Studio 2024, both installed on my desktop.
If you need me to test something on my laptop that I've done on my desktop I'll be willing to try it here.

All of this points to either vst3shell or Misstortion2 being the culprit; I'm afraid you have to ask their respective authors for help.

Dang it, I was hoping my problem would be easier to fix than I thought, thanks for the help though.
I have an idea of where to contact the developers of Misstortion2 but I don't know where I could contact the devs of vst3shell.

The KVR thread is the right place to ask. polac is the developer of vst3shell.

Actually, after seeing the latest post in the forum, it looks like this is an issue specific to v1.4 of vst3shell. I was still on v1.3.1, which works but I can confirm that v1.4 has issues finding plugins. You can download v1.3.1 from the first post of the thread instead.

Oh dang, that's a little disappointing that v1.4 isn't working properly with OpenMPT, that was the version I had installed for OpenMPT.
I downloaded v1.3.1 from the KVR thread like you said and followed the same steps I did when I installed v1.4, and I'm able to get Misstortion2 open now!
I hope polac finds out about the issue soon and finds a way to fix it, I was so frustrated trying to get it to work this past week I thought there was something wrong with me or my system.
Thanks for the help!

I'm not really sure what the expecation of vst3shell here is regarding how the host loads the plugin - it could be an issue in OpenMPT but given that it's not the only host behaving like this, I doubt it's OpenMPT's fault. Nevertheless, I will keep this issue open until I have confirmation from polac about the nature of the issue.