A - you would not need to buy more than 1 primary device, roku would definitely work as a secondary device
B - i thought roku and firestick would count as primary but could be wrong, are you sure you can't just check "i dont need this" on that screen
C - submit a service ticket when logged into the website. Tell them you really want to sign up but wanted to use roku as primary. I have a feeling they will find a way to accomodate.