This is an old thread ... you should probably open a new thread if you have a question someone can help you with potentially.
The answer regarding security is mostly correct. Common cable plant -- as is found in many apartment installations -- doesn't allow for any kind of endpoint security in a MoCA based technology such as FiOS. (well it could, but not in a way which would be scalable to manage such as a carrier would need). As such, they need to place a block filter on each unit's coax feed (which keeps your STB's from seeing your neighbors, and your LAN from being joined with your neighbors) which unfortunately also means that the signal for Internet also get's blocked (basically, anything above 1ghz). Thus the VDSL / ADSL2 to get an alternate path to bring the signal to your unit.
With traditional coax providers and DocSIS, the endpoint terminates at the modem (thus each user can get a seperate connection). However, you will also note that they never use the cable plant to redistribute the local LAN inside the dwelling (only ethernet and wifi) for exactly the same security (common cable plant) kind of issues.
I don't understand your performance issues however -- this is still a dedicated connection to you alone in this scenario and should not be subject to the the actions of your neighbors -- the FiOS feed for internet to the basement should be giving each unit a dedicated chunk of bandwidth across the VDSL feed. Have you done speed tests during these times of day (using the verizon speedtest site so as to stay on network) to see if the bottleneck is within FiOS or outside on the Internet at large? Would suggest that.