On the first question, any liability that remains at the end of a candidate's campaign becomes the responsibility of the constituency association. So if a candidate has a loan outstanding at the end of the campaign that they haven't repaid, it becomes the liability of the association. If the association were to be deregistered for any reason, its liabilities would become the liabilities of the party. There is a link there, but on the campaign itself, the candidate's loans would fall to the association.
On the second question, a candidate can receive a loan from a party or an association, but it has to be at a market rate. They cannot lend money at a commercial rate--depending on the terminology. If they were to lend money at a rate below the market rate, that difference would become a contribution and would need to be receipted and charged against the contribution limits.