Multiple clients listening the UART TX line
Yes, it will work, there is no difference between using one or multiple mcu's to do work as long as you can partition them properly so there is no contention/overlap between the work.

Since each MCU is only listening it should be quite easy to do(since they can never TX anything there is no contention on the bus).

So, you just have to specify some way for each MCU to do it's own set of work. Each each work load is independent of the other this should be relatively easy. If the workload cannot be split into independent pieces then it is a potentially much more difficult problem and gets into the realm of multi-threading.

Now, as far as the multiple receivers, that depends. Each MCU will act like a stub on the TX line and this can cause signal degradation(impedance matching issues, reflections, etc...). Each RS485 receiver will also act like a stub. Since RS485 is designed to do these things and has relatively low speed you should be fine.

It is difficult to know if this will cause problem or not because it depends on the fine details. How you route, bypass, length of traces, trace impedance, etc...

I would say, go for it, it will probably work fine.