Didn't know that about the Adam having different tape drives than the SGM. We always thought they were the same. Also thought in the age of the floppy disk (5.25) why a tape? High speed tape drive in a cassette type housing seemed like a disaster waiting to happen, which apparently did...
The original Tape Drive for the ADAM was to be the Entrepo Drive that was also being used in the SGM design. So in essence, the ADAM was not the end of the SGM seeing as Coleco was developing all these hardware items at about the same time... with the SGM getting a head start. Coleco's plan was to release the SGM as planned for the gaming market and then follow that up with the ADAM (Stand-Alone and Exp. Mod. versions), but all the difficulties they had with the Entrepo Drive made them ultimately nix the SGM and concentrate on a new Tape Drive solution for the ADAM.
Software wise, programmers probably would have included a test routine to see what hardware was being used and if the ADAM Computer was required, the program would error out. If the software was compatible with the SGM and ADAM, it would go on it's merry was and load.
While I agree it would have solved a lot of the Adam's problems, the Super Games would also likely have to have been scaled back a bit since the Adam's disk format was lower capacity than the tape format.
Yes, there is a difference in storage size between the Digital Data Packs (256K) compared to the 5 1/4 F.D.D. (160K), but ALL of the Super Games and other entertainment software developed and released for the ADAM by Coleco was under 160K in size and even much smaller. The reason you can't copy a game like Buck Rogers - Super Game from DDP to Disk is the way the programmers positioned the program on specific ranges of blocks on the DDP with large unused ranges of blocks in order to make seek and load times faster. Then block load routines are used instead of file loading.
Walters Software painstakingly disassembled all the Super Games and rewrote the load routines to squeeze everything down to fit on a 160K disk.
As far as Coleco going with a Single-Sided F.D.D., this was another cost saving measure for them, not for the buyer. They could have used Double-Sided mechanics and used either a 320K or industry standard 360K format, but choose poorly again.