Dunno. In my question, I'm only asking about the case where it is an unbeliever leaving a believing spouse: Does the "not bound" refer to the marriage i.e. the believer is not married and is therefore free to marry,
OR
is it (as I suspect it is) that the believer does not have to remain (bound) to the unbeliever when the unbeliever leaves, but is nonetheless not to remarry?

The young widows are permitted to remarry (1Ti 5). He never says this about those with a living (former) spouse.