Party affiliation as listed on voter registration isn’t a loyalty oath either. It’s just a way of asking which primaries you want to vote in. You might as well pick a party with relevant primaries in your area whether you agree with their platform or not.
(Voter registration is public information and may affect which candidates and PACs send you mail.)
Not nationally, but because Maine uses both Instant runoff voting for presidential elections, and the Congressional district method of assigning electors, it’s mathematically possibly for Maine to split its electors 2-2. eg, the Republican wins both districts individually, while the Democrat wins statewide. Not this year though – needs a competitive 3-way race so the runoffs matter.