“Gen edge 1 alternate” should work, if I remember correctly. In most cases, I was then able to mesh problematic blocks. Maybe it did work as intended, but your problem lies in other blocks? What is the error?
You can’t zone a block separately in 3DEC 5.2. All blocks are zoned together, when a generate command is given for each. You can however try to circumvent this by meshing a smaller model (less blocks) and then creating the other blocks. This way you can theoretically zone separately, but I don’t think this is recommend and only a last resort.
Splitting a specific block should work with the corresponding range command, but I mostly just hide all other and “show block XX” and use the JSET command without range.
Your questions suggest some deeper problems with the blocks themselves, maybe you can give a bit more info on them?