Unfortunately I am not familiar with the technologies you wish to use. you can use a powered USB hub to connect multiple devices to a single port, I believe, since they are likely all low bandwidth devices.
As i understand now, a Somfy Tahoma is an expensive bridge which i do not have, and RTS devices can be controlled by a much cheaper usb stick like ( nanoCUL USB Stick FTDI CC1101 433MHz FW 1.67 FHEM CCU / CCU2 Knick-Antenne FHEM CUL 433 + Adapter ) but i am just getting into this whole IoT thing and search for someone who already has this workling.
Never the less your right, i just fund some more informations here culfw-somfy-rts
I can’t help with most of the questions except number 4. No, home automation is a massive collection of closed protocols and technologies. For the most part you will likely need different transceivers for each technology.