Some rather minor updates to my problem of a DL DVD+R not being finalized properly.

First off, as I expected, 3.1.1.2 and 3.1.1.5 Beta behave the same as 3.1.0.5 Beta and 3.1.1.0 Beta in regards to this problem.

Following Portmac's advice more exactly, I was able to properly finalize a disc with ImgBurn by first finalizing the track and then finalizing the disc (skipping the step of finalizing the session).

I was rather surprised to find that a (SL) DVD+RW is finalized properly. I haven't tested any other types of media for this problem.

Something that does fix my problem (but may well cause other problems on some systems) is replacing the version 2.1.2.321 vso_hwe.dll in my DVDFab Platinum 3.1.1.5 Beta installation with the version 2.0.25.278 vso_hwe.dll from DVDFab Platinum 3.0.9.8, so I surmise that the problem has something to do with the way that vso_hwe.dll versions 2.1.0.311 and 2.1.2.321 interact with my system.