Q: When collation is not provided for a find/update/delete operation, but the server-side collection (i.e. not driver collection object) has a collation defined, I assume that applies by default.
A: Yes, that is the case.
Q: What if that collection has indexes with different collations? Might those apply instead if the query planner decides to use such an index?
A: An index is only eligible to be used in an operation if it the index collation is compatible with the operation collation. For example, an index with collation X cannot provide a sort for an operation with collation Y. If an index is not eligible, it is not considered for the query, but the query otherwise proceeds as normal.
Q: Do indexes with custom collations require manual hints to be utilized? And in that case, I assume they'd only be suitable for find, since update/delete do not support hints. According to db.collection.deleteOne(), it doesn't appear as if indexes with collations would be used.
A: Hinting an index with an collation incompatible with the operation collation will return results, but it will be inefficient and use wider bounds. This is the same as if you hinted an index {b: 1} for a query operation with predicate {a: 1}.