UPDATE! Problem is fixed! I repeat, I solved the problem!
It turns out that one single addonped has been causing all my woes. I removed it and viola, everything works as intended. However, it took me two full days to come to that conclusion but whatever. In regards to two of Stewarts vehicles, well... that one's been a bit of a ballbuster.
SO, I'll start off with Stewart's vehicles because that was what I solved first. I researched the "generic error" I got from the two vehicles, that generic error being "ERR_FIL_PACK_1". I looked it up and it turns out that I needed the Packfile Limit Adjuster in order to fix. Several days ago I uninstalled that because I thought it was the one preventing me from loading MP maps (as it said in the description not to do so). I installed it and it immediately fixed Stewart's vehicles, so we're good on that front.
However, the AddonPeds took me a whole day to fix. At first I was trimming the file size (which was time consuming due to how RPF files worked for some strange reason) to see if that was the problem, but it wasn't. I cut down the file size from 2.3 gbs to 1.2 gbs, and no dice. So I figured "it might be one or more of the peds causing problems" so I uninstalled them all. The game loaded without script errors when I did, so it further reinforced that theory. I installed the peds I wanted to keep in batches of 5 to 10 (with all of the peds highlighted so I know which ones were the potential problem ones). 90 peds in, the scripts crashed. I then installed each ped one by one, and lo and behold...
It was fucking Neo. It was the Neo addon ped.
That single ped model has caused me grief over the past five days. After deleting that ped, I installed the rest in batches, and found no issues. No script crashes, and nothing in regards to Addonpeds crashing from loading peds that came after Neo (that was a problem in of itself but I had a stopgap measure already in place, thankfully this is no longer needed). Everything works as intended. So yeah, I can finally rest easy.
TL;DR: Some vehicle addons require the Pack File Limit Adjuster for some reason, and the Neo Addon Ped causes multiple issues for unknown reasons.
Thank you all for your time and viewership. Hopefully this helps someone else later on down the line. Good night to you all, and happy modding!