All telephones can be connected via the analogue telephone connection.
If you want to connect the handset directly to a Fritz!Box DECT base, it should support the CAT-IQ protocol so that all functions work.

Should work without any problems. I have connected mine to the Salt Box. Plug it in, press the connection button, done

Yes, it should work on one of the rear connections FON 1 or FON 2, but not on the ISDN port. The specifications and protocols of the connections are standardised to enable interconnectivity between products from different manufacturers. I don't know whether you have to set anything else in the menu of the 6490 apart from plugging in the connection cable. This was not necessary with my UPC Connectbox; I even connected 3 different telephones in parallel and it worked straight away. Incidentally, I am very satisfied with the Comfort 500. In the unlikely event that it doesn't work, you have the right to return it.

I don't know Fritzbox. However, I was able to connect the phone to the Sunrise box without any problems.