Q. 1: no, the switches (A) do not need to route at all, that's what the firewalls are for. The only exception would be if the modems are located in a different subnet (but that is improbable).
Q. 2: I would point all switch gateways to the firewall cluster address. Whether they can reach the internet or not, and by which kind of service, is then under your control (via firewall policies). Typically, you would create a management VLAN (in your case, two) which terminates at the FGT.
Q. 3: Drop switch B if at all possible. Even if locally separated, the HA link between Fortigates should not run across active devices. Technically, it can, but the risk of a HA link failure is way higher than with just using wires (fibers).
And provide for 2 HA links for redundancy.
Q. 4: why VRRP? FortiOS HA provides all you need for sub-second device failover, active link and line monitoring, even load sharing. In my experience, VRRP takes ages in comparison to fail over, and would only be used if you had hardware from various vendors.