I have not so much trading experience to give a general answer - it also depends a lot on the strategy.

Training too often has the disadvantage that the parameters change too often, and that can have an adverse effect when series suddenly change and open trades are continued with changed parameters. You can emulate this by using a very short WFO test period - then you can see how this affects your system.

As to the second question, different pairs normally require different strategy parameters. I'd say that it is indeed an indicator of robustness of a strategy when it returns profits in a WFO test with parameters generated for all pairs together. But I have not tried this so far.