Switching 15V, 1.5A AC with SSR

It might be on the optimistic side to expect the resistive life to apply to a CCFL load, however an overrated (eg. 10A or 16A) mechanical relay with any kind of a rating for a tungsten load will probably last just fine with a 0.5A CCFL load.

If it was purely a resistive load you'd probably get in excess of 10^6 operations. Check the life curves in the relay datasheets.

You do not need to place two SSRs in series to your heater. As a matter of fact two SSRs in series probably do not function very well any way.

Instead you should incorporate the flow detector output and on/off control switch logic into a common signal control with the single SSR that the PID controller operates. In all likelihood the PID controller itself may very well have an enable/disable input that can be used to support the extra shutdown logic.

If there are no extra control inputs to the PID controller then a simple small signal relay can be used to interrupt the control signal to the PID SSR.