I just read this thread, and it's very interesting. There is some great advice and diagnosing going on.

Let me take your questions in reverse order and then offer a thought in another direction.

3) Yes, you can. One of the folders could be /BB as it is now, and the other /BB2018 (or vice versa). The files in common that are NOT in the BIAB folder, like fonts, haven't changed to my knowledge.
Ideally, your computer has enough room for complete installations of both. If it does not, we can talk more about going into Preferences to select the same location for your RealTracks and then RealDrums. This could be problematic for the 2018 version not recognizing some sounds, and/or not indexing them.

2) Yes, I think so, but I haven't tried it since I update every year.

1) Not sure about this one. However, following Simon and RHarv's line of thinking, I want to suggest something else. How powerful is your new computer? The minimum requirements to run BIAB successfully have increased somewhat. A "gaming" computer means to me that it has a powerful graphics capability. That doesn't say much about RAM, disk storage, or the CPU, and the ONE thing BIAB doesn't need is a powerful graphics card. Since you do not have an external audio interface, it makes me wonder about the rest of your hardware. Perhaps something that worked in version 2018 now stresses your computer enough to produce unwanted noise. I do highly recommend what others identified, that you should get an audio interface. There are several inexpensive USB ones by Presonus or Focusrite. Any of them would be better than the internal RealTec chips.

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.