At first you said that he was "more or less convinced" that he was lax, but then later stated that he wanted the priest to think he was lax, implying that he deliberately lied about it. Sounds to me more as if he was uncertain. But that is precisely the hallmark of scrupulosity, the inability to form a practical judgment ... so the scrupulous always spin ambivalence as certainty that he had sinned. So, in his warped mind, being unsure that he was lax, by telling the priest this, he was lying.
As to whether it invalidates the Confession, it would either 1) have to bear directly on some sin confessed (usually to make it seem less serious than it was) or 2) have to be done deliberately so that there was no intention to repudiate sin (how can there be when you are intending to sin). But my guess is that, as per the usual modus operandi of a scrupulous person, he's deciding after the fact that he "lied" when in point of fact he was uncertain ... and the scrupulous spin on uncertainty is "If I wasn't 100% certain, that means I lied."
So, this is indeed where the arrogance of the scrupulous is their biggest obstacle. They absolutely refuse to listen to people who tell them that they're scrupulous. Instead, they decide that these others are lax. Here, by telling the priest he's lax, he's going to get the exact opposite of the advice he really needs. But that's what he wanted. He secretly wants confirmation that what others call scrupulosity is in fact the true reality (because his pride makes him cling to it).
Bottom line. HE IS NOT TO DECIDE WHETHER HE IS SCRUPLOUS OR LAX. That must be left entirely up to the priest to decide. And your husband MUST SUBMIT to the priest's judgment. Period. If he refuses to do so and to simply resign himelf to the judgment of the priest, he will end up losing his soul over it.
There's some truth, unfortunately, in his assessment that he does not love God enough. He's absolutely self-absorbed. He's got to focus on loving God and not on sin and punishment and his own personal state of soul.
As for not praying because he thinks he would be distracted, distraction is only sinful when it's completely deliberate (the chief problem scrupulous have, discerning whether something was deliberate). Deliberate should be understood by a scrupulous person as: "I'm saying the Rosary and I turn on the TV." ... so some very deliberate intent. Otherwise, the scrupulous should no be allowed to get away with fancying every inability to dispel distraction as being deliberate. If the prayer is not required (i.e. such as assisting at Mass), it is better to pray distracted than to not pray at all. There's a story I heard from a Jesuit. So, a Jesuit asked his Confessor, "Is it permitted for me to smoke while I pray?" His Confessor said, "No, of course not." So this Jesuit then went to a different Confessor and asked, "Is it permitted to pray while I smoke?" This time he gets the answer, "Of course. We are to pray always." So, if the prayer is not required, such as assisting at Mass on Sunday, it is better to pray even while distracted than to not pray at all ... as per the second category.
At this point, since he's obviously refusing to submit to the judgment of his confessor, the only remedy for scruples, I think that your intervention is required. I think that you need to go with him to the priest, make an appointment, and tell him everything you've written here. Your husband can then try to argue his case that he's not scrupulous or whatever, while you give details to the priest about your perspective on the matter. There's no doubt but that the priest will decide that he's scrupulous. This would work similar to marriage counseling, where both present their perspective and let the priest try to make a determination regarding the objective reality. Then your husband must submit and accept the fact that he's scrupulous. He must submit to the priest's judgment. This priest needs to command him never to refrain from the Sacraments unless he can swear before God that he committed a mortal sin. Earlier when your husband implied that the priest needed to command him "under pain of sin," that speaks again to his arrogance. No priest would add another layer of "sin" on a person who already thinks everything is a sin. So he would then struggle with whether he sinned by disobeying the priest. It would be an infinite loop. He simply has to submit, without it being required under pain of sin.