Odoo 11.0+e (Enterprise Edition)

I've been having a few problems with trying to correct some errors. 

Firstly, some expenses have been posted which haven't been actually been paid from that bank account. I have tried every possible way to delete the posted entries but I can't do it. I guess this is with good reason but not helpful when it was a mistake! Is there a way to get around this?

Another issue is that some invoices were marked paid, then open, then paid again - possibly due to getting the date wrong first time. This has meant that there are duplicated journal entries and therefore the cash book balance doesn't match with the bank. I had been deleting duplicated entries with some success, until I noticed a couple reappearing in bank reconciliation again. When I tried to reconcile the entries, this warning appeared: “Operation not allowed. Since your statement line already received a number, you cannot reconcile it entirely with existing journal entries otherwise it would make a gap in the numbering. You should book an entry and make a regular revert of it in case you want to cancel it.” How do I fix this??

Sorry about the lengthy question, would be very grateful for any help!