Believe it or not, 19 February 1722 comes AFTER 25 May 1722. That is because, prior to 1753 in IOM, the calendar ran from Lady Day (March 25) to Lady Day. Thus, March 25 was the first day of 1722 and the last day was 364 days later on March 24. This is known as the Julian Calendar. It was switched to the Gregorian Calendar (Jan 1 to Dec 31) in 1753. I believe it was 1752 in England. I refer you to Frances's Manx Notebook (MNB) section A300 Old Style & New Style Calendar. You can find it easily by searching on "Calendar". So, William's widow was truly a widow when she married Nicholas Cowin 19 Feb 1722/23. The "1722/23" is the accepted way of documenting the period Jan 1 to March 24.

As far as the "his" children and "her" children in the 1722 will of William, and since I was the transcriber, I will dig up the original photocopy that I transcribed to see if I made a mistake. Unfortunately, all these documents are stashed away in a cabinet which is out of easy reach so it might be awhile before I can get at it.

It is interesting that Catharine Cowin formerly Kermode als Quay will 1743 mentions her son Thomas Cowin under 20 years, and the IGI shows a child bapt in 1730. Her first husband William's 1722 will mentions (as you say) "her" sons John & Thomas. Unless she had two Thomas's with different surnames (it does happen), it would seem the first one must have died. Catharine's will also mentioned her son William Kermode and his wife (unnamed) but didn't mention a son John. This will isn't transcribed nor is it in MNB but I did an extract 25 years ago from a film which I'll include below:
Lonan Will Catharin Cowin als Quay 27 Mar 1743
Son Thomas Cowin