Ok, I think I understand 
For the most part, you can simply tag the payment to the solicitor to the invoice from the supplier. The fact that it’s paid via an intermediary doesn’t really affect your accounts so this way should be OK.
With the loan from the supplier, you would need to set up a loan account to keep track of the amount owed. Assuming the loan is for the balance of the invoice, you can simply create a loan type bank account, and start it with a money out transaction tagged to the invoice. There’s an article here about credit cards, but the concept is the same.
I hope that’s clear, but please don’t hesitate to ask for further help