Purchasing wifi 6 model RBK753S-100NAS at costco

I can't answer #1 about compatability with other satelites except to say the RBK753 has Wifi-6 and the older Orbi's don't. Not sure if it can work if you turn off the 802.11AX functionality.

#2 has been answered, so on to #3. I have liked having Armor, enough so that I paid to renew it. It gives me right to install Bitdefender Total Security on all my Android, IOS, and Windows devices and in addition protects everything connected to my network. Now there have been some false positives, and I can't tell you if there have been any false negatives, but all in all I would need a decent solution for my windows devices (not ready to trust Windows Defender fully yet).

The nice thing about Costco is they will let you return it if it breaks or just doesn't meet your needs.