Learn English – Is “widower” the only word that adds a suffix for men

I suspect because the phrase was only needed for women and widower is a much later literary invention.

Widow had a lot of legal implications for property, titles and so on. If the survivor of a marriage was a woman things got complicated before women had many rights. If the survivor was a man in the middle ages it didn't really make much difference as he held all the property anyway.

A similar question came up about illegitimate girl children, there was no word because there was no legal need to consider them.

For the rest - English generally doesn't have many genders anymore and those that have survived are where it was necessary to know the actual sex. So for example "actress" once had rather more of a euphemism role (like the modern 'model actress whatever') — where knowing their sex is relatively important.

It is, as you say, a nominalising suffix, that goes back at least to Common Germanic. It usually surfaces as "-th" in English, but as "-t" after a (historical) "-(g)h": "height", "weight", "sight", "flight".

The vowel alternations are mostly a matter of length (though I admit that the "-ong"/"-ength" alternation sounds a bit like IE ablaut). "Wide" in Old and Middle English was bisyllabic, with a long vowel, whereas "widþ" had one closed syllable and therefore a short vowel. Only the long vowel went waltzing round the mouth in the Great Vowel Shift. Thinking about it, this particular alternation may go back to a time when the IE ablaut was operating, but I'm not sure.