I think it's highly possible that your husband and the OW have been in contact throughout your conversations with her and that they've agreed a 'joint story'.
I sort of 'get' that stuff you said about the imagination often being worse than the reality, but I remember posting something once on here to a woman who was having an affair and I talked about this a lot to a mate who was in that position. I know you're on the opposite end of this, but hopefully it will still resonate.
Truth's a funny thing. There are factual truths i.e when, where, what happened and then there are belief truths which are massively open to interpretation and can get distorted to fit the belief system of the person telling those 'truths'. So this means there are factual lies and/or belief lies.
Your husband has told you lots of 'factual lies' so it means that you are nowhere near even scratching the surface of his 'belief lies'. So for example he's lied about the OW 'blackmailing' him throughout the affair and while this goes unchallenged by either him or you, it allows his 'belief lie' that he wasn't responsible for his actions and that the OW was the villain of the piece.
The OW will be in a similar place and although because this is you she's speaking to and not her husband, she might not be telling 'factual lies' (that is unless she's in cahoots with your husband) but she will have told you lots of 'belief lies'. I can imagine for example, that you will have heard all about her 'low self esteem', her difficult marriage, her loneliness and sadness about some event or another. But rather less about this being typical of her selfish character and that it's also connected with sheer lust and/or sexual frustration.
The advice remains the same. Ask him to leave and then see how you can manage life and your disabilities without him. If he keeps being allowed to stay when he's found out in yet another lie, he'll just continue to do it. You're more likely to get the truth when he thinks it's over and coming clean couldn't make anything any worse than it already is.