Oh dear! I seem to have laid myself open to a barrage of grammatical quirks. The cases cited, I suspect, arise from the same source as the mishmash that is our spelling, the lexicographers of the eighteenth and nineteenth centuries. Possibly “hisself” was rejected on grounds of euphony and the plural form followed to match. Certainly the “ungrammatical” forms are still in common usage here in the West Country. What I meant by grammar was the function of nouns, verbs, adverbs, adjectives, gerunds etc. and their interrelations.
Having said that, I observe that it has all gone to pot in the last few years. Nouns and verbs seem to have become completely interchangeable (My bad!, to big up etc.) particularly in blogs that adopt the North American idiom (such as Climate Audit) and trendy columnists (usually female) in the UK. I personally find it all rather tiresome, but then I am old and irrelevant. Combined with the modern tolerance of multiple typos and the easy adoption of neologistic acronyms, this, I find, makes reading on the internet hard work.
But the internet seems to be changing language all over: tutoiement, for example, seems to be universal in French forums; unheard of not long ago.
I grow old. I grow old. I shall wear the bottoms of my trousers rolled.