OP, one parent missing trick or treat while the other parent endures enjoys it is no big deal at all.
Getting a neighbour to check sounds quicker than him coming home to check for an intruder, and in that circumstance quicker was better. And once checked, why come home?
If you are part of an orchestra, amateur dramatics group, chamber orchestra, any activity where your presence is important to the functioning of the group activity, it IS a commitment. And you have known he is part of this since you met him.
It was quite major to cancel the whole group’s performance at a fete or whatever in deference to the chance of your catching Covid before your holiday, and yes, that would look bad if they had committed to performing.
And if the rest of the troupe (group? Band? Cell?) were busy because they had committed to a typically Morris-orientated event such as May Day then they would be disappointed not to be able to attend his wedding.
Are you jealous of his commitment and time? Is it because his ex was / is involved?
How much is about your insecurities?
Is your Dd his Dsd? Did you spend years as a single parent when it would have been impossible to have such a commitment outside of your parenting, and this has either affected your view of what being a parent and family is, or is making you jealous?
Is there a high level of troupe members having affairs ?
Does it all involve drinking?
Are you jealous or envious of the bond, the shared connection and group culture that I expect Morris has?
Love it or loathe it, Morris dancing does add to our culture and community. He does it for his enjoyment but it also provides enjoyment and entertainment to others and for community and cultural events.
I hope you manage to work through this in a way that makes you all happy.