Churches are active places of worship - not handy venues that look pretty in pictures. The churches are only still open because of the worshippers who support them all of the other days of the year.
Indeed. The OP said they had a connection to the parish, but not the actual church. If they had had a connection, none of this would have happened. The family (the OP's or her DH's) would have had a word, paid in cash, and the OP reimbursed them. I've seen this - pretty common local practice.
The OP says her DH went to school with the church warden's son, but that is not really a connection to the church or the congregation! Everything about this saga suggests that they were pretty much strangers picking a nice church in the town/village which happened to be near where the DH grew up.
Look at it from the church treasurer's point of view: these are people whom he doesn't know, they're not regular members of the congregation (nor their families either, it seems). He'd sent the invoice, specifying payment before the couple were married, and they had questioned about how to pay. But not actually paid.
Then they were married, still without actually paying the stated set fee, then they were uncontactable for 10 days, still hadn't actually paid the stated set fee.
The treasurer felt responsible for the fraud which he saw committed.
As the couple were pretty much strangers to the parish, how was the treasurer supposed to know they weren't intending not to pay? All their actions (as opposed to their words) pointed to them not intending to pay. No attempt to get a bank-produced cheque, or paying in cash, or getting someone in the family to pay by cheque. Nothing.