What if Joanna the Mad marries Charles VIII of France, what will be the effects of a temporary Valois France-Spain personal union and a Valois Spain.
Only if Joanna's elder brother Juan dies young as on schedule. And the Habsburgs and the Trastamaras both had very good reason to oppose France, as it threatened their Italian ambitions.
Exactly. The whole point of Joanna's marriage to Philip - and the marriage of Juan and Philip's sister Margaret - was exactly to create an anti-French union. I'm not sure why they would change policies here.
I think this would or might result in a war of succession like what happened in OTL.