As a native Dutch person and a teacher of English and a former teacher of German, I am grateful that at least English grammar is far easier to learn than German grammar - none of my German students managed to speak the language AND use the correct case endings or verb forms, whereas all my English students can compose grammatically correct English sentences.
Also, it is really hard to explain why a German or Dutch finite verb should move in front of the subject in one situation and to the end of the sentence in another - much easier for everybody to remember that English sentences always have Subject, Verb, Object - in that order. Always? No, there are some situations where English has retained the Germanic tendency to move the verb around, but luckily in MOST cases, modern English keeps to the 'simple' S-V-O structure - as, by the way, do the French. And this is no coincidence: from 1066 onwards, Norman French was the official language in Britain for hundreds of years. That influence did not disappear when they started speaking 'English' again. While we were taught that English is a Germanic language, two thirds of its vocabulary today originated from French or Latin, and the sentence structure is also far more like French than German, down to the distinct forms for adjectives and adverbs, which de Roman languages French, Spanish and Italian all have, while German and Dutch do not. In this area, the general tendency towards simplicity can also be noted: many Americans tend to say 'come quick' rather than 'come quickly', and if they keep doing so, sooner or later that may be regarded as okay.
An example of how the Roman influence 'simplified' English grammar can be found in the verb forms: while many of the most frequently used 'Germanic' verbs in English have irregular forms for past tense and past participle (begin, began, begun/ know, knew, known), ALL the verbs based on French or Latin origin are regular: To comprehend, comprehended, comprehended, respond, responded, responded, demand, demanded, demanded. Why? well, no one would have known which of the many different forms of the irregular verbs one should have applied to any of these newcomers, and besides: why make it more complex than necessary?
This might raise the question why so many Germanic verbs are 'irregular' in the first place - I don't know the answer, but I guess it is because they originated at a time when language was not written but only spoken: the vowel changes provide much more audible distinctions between the tenses than just an added 'ed'.
While the contacts between the various languages seem to have led to the adoption of whichever grammatical form was simpler, the influx of words from French and later from Latin gave English a very rich and complex vocabulary, with an abundance of synonyms in 'high' and 'low' registers and highly specific terminology that is perfect for academic distinctions. A maximum of content conveyed with a minimum of rules - quite perfect for teaching, really! which for me is at least one of the explanations why English was and is so tremendously and unprecedentedly successful as an international language in all areas of life.
As far as I am aware, pronunciation is not part of grammar - but apart from that, it is not English pronunciation that is 'a mess' but the relationship between spelling and pronunciation. This 'mess' results at least in part from the fact that English pronunciation changed (the great vowel shift, see Wikipedia) while spelling did not. Besides, lots of British words owe their spelling to French, and French spelling is notorious for its abundance of vowels and consonants that are not pronounced at all. No wonder some of the superfluous vowels were omitted when the American spelling was standardized by Noah Webster in 1783. But by that time, the Americans were already convinced that the 'natural' way to spell the sound [i:] is 'ee', even though that conviction would be contradicted by the spelling of 'grief', 'thief' or 'belief'. Anyway, most spelling issues are easily solved with a spell checker. Although in this very text, the spell checker wants me to change 'their spelling' to 'they're' or 'there'...