What is 'only' is these sentences?

Solution 1:

Here, it's a pragmatic marker showing deference (a hedge). Conversational oil, meant to tone down any suggestion of pushiness. It can be considered as roughly equivalent to 'It's just that ...' or 'I'm only asking because ...', but these latter are heading more in the direction of subservience.

I'd not try to label it as 'coordinating conjunction' or 'adverb class 847', but it does stand apart from the matrix clause/s.

I'd love to use a dash here, but the question mark needs to go after the question (and ? – might be considered ugly by some; they'd probably pontificate that it was unacceptable), so I'd use your two-sentence version.

There could well be situations where confusion with the 'just' [focus particle] sense of 'only' would be more likely.