sorry for ultra long late reply
One thing I can say is, ImgBurn doesn't support multisession burning at all.
this is the reason why your disc doesn't work - DC just won't recognize single session disc as game disc.
I made some search, and found only way to write multisession disc is cdrdao, cdrwin, or clonecd
theoretically, you can made some hack with clonecd's image file and toc file to write multisession discs... I'm still researching
from your result, I think you just didn't made a multisession disc. try to write a multisession disc properly, and you may get some new results

I don't know is this on topic or not...
I'm looking for a way to burn GDI image back to CD-R, and I tried to merge those tracks back to a DAO image.
After I inserted enough blanks after track 2, merged all tracks together and wrote a CUE that indicates Session 2 starts at LBA 45000 (I know there is no way to burn this image back to CD-R, but just for researching.). Suddenly, IsoBuster recognize the file system! All files can be extracted directly... Maybe it's really an old news, but why IsoBuster can't recognize the file system when I open the data track directly?
And I made some basic calculation, starting from idea: How if I burn a GDI image by using hybrid method of GigaRec?
Assume I'm using 90min disc, Burn 1st session in normal mode, and burn 2nd session starts from LBA 45000 in GigaRec 1.4x.
90min=405000, 405000-45000=360000, 360000*1.4=504000, 504000+45000=549000. Very close to original GD's 549150 sectors... If I burnt it successfully, will DC recognize it as a GD-ROM...?
Currently I don't have a Plextor PREMIUM... I missed last yahoo auction (3 days ago... bid at 1100 yen), waiting for next shot. And my Dreamcast is at MY home -  I'm (living) at Japan but MY home not.