As for Sam asking questions.....I suspect the only thing Sam would want to know was how much he was going to get paid for this mystery job....and how much booze he could purchase with the payment.
I agree with Rainey here. Also, when Barnabas first approached Sam about having the portrait done I do recall Sam asking questions and thinking the whole matter rather strange. But I guess with the offer of $1,000 Sam would probably go along with just about anything.
As for who painted the portrait of Angelque, perhaps the DuPre family paid for her to have it done. They did seem rather on close terms with her and it could be that when Josette had her portrait done the Dupre's decided to have Angelique painted too.
On the other hand, it's quite possible that Barnabas paid for her to have one done when he sat for his portrait in 1795. This could be one of the reasons why we never saw the portrait in 1795. She couldn't very well say that Barnabas paid for it since it would give their "steamy love affair" away.
Cassandra