First Question: No. There is not, nor has there ever been, a specific dollar amount that the universal Church set to distinguish mortal and venial sin; though it is very possible that some (or even many) priests set an arbitrary amount and told people this in confession. Frankly, I cannot imagine a priest telling someone that stealing $40.00 would not be serious sin today let alone in the 1950s and before when $40.00 would be worth so much more than it is today. Consider the fact that $40.00 could easily be an entire day's salary at $5.00 an hour!
Second Question: Examples such as this is what demonstrates the insidiousness of the Conciliar church. I am not sure that anyone on a forum such as this can truly answer the question. I can certainly say that the safest answer is to say that man should not assume he can marry again. Virtually no person, even if he accepts--hook, line, and sinker--the heresies of Conciliarism believes that he has left the Catholic Church. Christ warned about following blind guides because both end up falling into the pit. Also, remember the Catholic Church recognizes marriages of non-Catholics as valid. The Pauline Privilege applies only when conversion to the Faith occurs and the non-converting party refuses to accept the conversion and rejects the convert which is not the case here as the parties separated before the man's conversion.
But even directly supporting the heresies of Conciliarism does not mean that he did not intend, at the time of the wedding, to fulfill the requirements of marriage. He may very well have approved of his wife's use of birth control pills, but did he truly intend and expect that he would never children and absolutely reject even the possibility? Just because one believes that contraception is acceptable before God, that incorrect belief can lead to sin and it can lead to leading others to sin, but holding that, or any other, heretical belief does not, in itself invalidate a marriage.
Furthermore, even Conciliar catholics generally seem to enter into marriages with the intent that the marriage will be until death. In the first scenario, I don't necessarily see, on the very few facts presented, anything that would mean the man's first marriage is not valid and can be put asunder.
The second scenario, i.e., the civil wedding, may be a different story since he was not even married in the eyes of his own sect.
In any case, due to the lack of a Catholic hierarchy in Rome, I think you may get different answers from different priests.