I think you've definitely got the right idea with integrity and honesty.
I'm hoping that this yellow flag of her perhaps saying she and her husband were more along the poly train than they are, plus telling you that she could be naughty with you before he's ready, is just because she's new to poly and she's getting carried away with the excitement.
I would suggest taking it very slowly and perhaps cutting down on the time you spend together... or perhaps pick specific types of dates and times that will make sleeping together difficult.
I'd also suggest getting to know the husband more and talking through things with him.
Obviously, whilst he does not own her, she should really be patient and considerate of him. She needs to let him know if she can't handle waiting any more. It's her responsibility to get her needs out there... it's not fair to act on them regardless of his feelings.
For example, my GF recently started dating. It was going to be her first sexual partner outside of her marriage and myself. She had one date with the 7th guy she met, came home and said to me "I cannot wait any longer for this, I'm chomping at the bit, I need to do this now. If I have to wait longer, I'm afraid I'm going to break a guideline. I don't want to do that, but I've waited long enough."
I found this a bit offensive at first, that she couldn't control herself. But then I realised, she just couldn't wait. I might have a little more self control than most people - that was her limit. She knew that for her, waiting any longer was going to cause resentment and unhappiness. I think that your potential-new-lover really needs to get her needs across to her husband if they are that strong that she's willing to act now.
How long have they been exploring the idea of poly in theory?