First off, realize that Circuit breakers and Fuses perform the task of protecting the CIRCUIT, i.e. the wires, from fast acting short circuits such as a wire going to ground or one motor winding melting into another one. For this reason, CBs and fuses collectively are referred to as SCPDs, Short Circuit Protective Devices.
The Overload Relay protects the MOTOR from a long-term running overload. It does not see the current from a short or ground fault until long after a fire will have started. It also does not interrupt current, it just senses it and trips a little relay contact. You would then wire that relay contact into the coil circuit of a magnetic contactor to drop out the magnetic coil power if the Overload Relay trips, and it is the contactor that interrupts the current flow to the motor. The Overload Relay works by sensing the heat produced by the current flowing through the circuit and uses the I^2t (Inverse Current / Time) calculation method to determine that heat. So an overload Relay is referred to as a Thermal Over Load (TOL) device.
According to the NEC every motor circuit, however it is used, must contain a SCPD, a TOL and a switching / controlling device, as well as a means of safely disconnecting the motor from power and locking it off.
Technically fuses can theoretically only be used as the TOL on motors 1HP and below, but only if they are specifically sized to a narrow range of the motor FLA, 115% max. I believe. The problem is, fuses only come in certain sizes so it is not really practical to find the exact right size, plus you have the problem of what happens when only 1 fuse blows. This however does not apply to your situation.
A circuit breaker is similar to the contactor in that it has all 3 power elements linked together and opens all 3 at once, but it is not automatic in that it does not (typically) have a "coil" circuit that can be operated by the Overload Relay.
So in your case, I would replace the 30A fuse block feeding the idler with an Overload Relay, and then use the NC contact of the OL relay in the 24VAC coil circuit of that 90A relay to drop it out if the idler gets overloaded. The caveat to that is the size of wire you have used. Since your upstream fused disconnect is sized at 50A, you can use that as the SCPD for the entire circuit only as long as all of the wires downstream are rated for 50A. If not (which is your case), the OL relay is considered to provide long term protection of the wires when combined with upstream fuses, just as long as 175% of the motor FLA must does exceed that fuse rating. So since your motor is rated 28.5 FLA, 175% is 49.875A, tantalizingly close to the 50A fuse rating. I personally would go for it, but you have to take your risks as you see fit. The safest route would be leaving the 30A fuses in there and just adding the OL relay downstream.