It can. I simply had a problem when I tried to do "verify written data" with K3B.

I can't say anything about 100 gb, I didn't even know they were available in that size. But M-class discs can be written with it. I do that too. Software is included for burning

According to the data sheet, it should be able to do this. However, I have never tried it due to a lack of specific media.
Any reasonably up-to-date burning programme should actually work for burning. Since M-Disc corresponds to the DVD or Blu-ray specifications and only the coating of the writing layer is different.

I never need the appliance. It turned out to be a bad buy, but that has nothing to do with the quality.

Unfortunately, I don't know. I use the device very rarely and really only for reading. It's practical because most PCs today no longer have such a device built in.

Oh, challenging questions. I would try asking these questions to Digitec support, they can clarify this with the supplier.