The parties have talks before the vote, and they make clear their intention of supporting or not this or that candidature so there's usually no surprises the actual voting day. I think a rejected candidate has another opportunity, not sure. When there's no more (reasonable) candidates left the elections are called for by the assembly itself and/or the king (even tho arguably in an honorary role, everything goes through the monarch).
Feijóo, even with more seats, has it harder. Only vox would touch them with a ten foot pole. Sánchez has it hard too, needing to assemble ALL the other groups, but this is more likely than the right getting the support of someone left from PP or the regionalists.
It's gonna be a few months of this pacts, what-aboutism, and other politics bullshit...