Grouping and paging CPT events by month with custom field date

This is not something that WordPress can easily do. You can think of pagination in WP as sliding window that shows fixed size part of one large query. It can be at start of the query or at the end, but it relies on it being same query.

This doesn’t nearly work when the size and number of the page are subject to variable logic like this.

I had recently answered very similar question and recommended to use date archives. But your case is more complicated since your effective dates are in custom fields.

I would guess overall you won’t be able to easily built meaningful pagination-like navigation for these. Something like faceted search might serve this better.