Well, first of all it would require harsh conditions. A union of parts of the Netherlands and England would strengthen the country quite a lot, so I propose 4 scenarios, started by an event with 4 options which I will explain later:
-Humphrey claims the English throne, with support of his wife, but they get beaten by the english.
requirements for ending: Holland controlled by the English
effects: Holland annexed by the English, but no cores etc.
-Humphrey claims the throne, and he wins.
requirements: Holland controls Anglia
effects: Holland annexes England, becomes England and adds all of Englands cores and cultures, possibly also the French ones if England wins the HYW. (not necessarily in this order)
-Humphrey stays neutral.
requirements: none
effects: none, except that all other events aren't woken up.
-Humphrey supports a pretender to the throne.
requirements: this pretender wins the war
effects: alliance Egland Holland, relation boost, first holland loses some money and maybe troops, later gets back more IF this pretender wins.
The event options would be:
-Claim the throne:
Temp. cb on England
War with england
-stay neutral
nothing, maybe +/-1 stab
-support Lancaster:
-200 ducats
lose 5k infantry
-support York:
idem.
Likely not entirely hisrical/logical etc, so feel free to comment.