[felxh]

I think you are correct that english grammar simplified because of the intermix with different languages. For example during the viking conquest, many viking words were very similar to english except or their grammatical endings, so people dropped them.

On the other hand, I think the irregular spelling has more to do with English never undergoing a mayor spelling reform: many times, the spelling reflects an old pronunciation. From what I know, England always had a relatively lax and pluralistic approach compared to other European countries when it came to managing its language. For example in France you have the Académie française, which essentially has authority over the French language. There is no comparable thing for English, which is probably why such reforms never took place.

Also, correct me if I am wrong, but you seem to incorrectly assume that English (or at least English grammar) stems from Latin.