Hi,
It all depends which SRX is primary at the time the traffic arrives. In your diagram, Node0 is primary and Reth0 and Reth1 are also primary on this device so traffic flow is as follows:
Router B -> L2 Switch 2 -> L2 Switch1 -> Fw Node 0 (Reth1) -> L2 Switch 3 -> Server
IF the link between L2 Switch 1 and 2 is down, Reth1 should failover to Node 1(provided weight etc is configured accordingly) and traffic would flow like so:
Router B -> L2 Switch 2 - > FW
Im not sure how your server is set up but if say the link to L2 Switch 3 fails, then yes, the server will be inaccessible. If your switches were in a Stack or Virtual chassis with the server NICs in a HA config (team for example), then you would have no issues.
You could also look into Upstream Device Failure Detection.