Out of all of the languages in the world, English is the best at accommodating foreign vocabulary and grammar

That is not true and no-one with a linguistics background would ever make a claim like that. It's just not how languages work.

Partly you're talking about phonotactics - which sounds are allowed to go together and where in a syllable. But more restrictive phonotactics doesn't make languages worse at loanwords.

Over 50% of japanese, korean and vietnamese words are chinese loanwords, despite very restrictive chinese phonotactics. Aisukurimu for icecream is because "skr" is not phonotactically permissable in japanese and syllables must end in a vowel or 'n'.

English heavily distorts loanwords. 'H' cannot be in the middle of words so a lot of middle eastern loanwords get warped. Constant clusters that english doesn't allow get a short æ like the first two Us in aisukurimu, except they're not written and most people don't realize they're doing it. Some sounds are just ignored - pneumonia, psyche are greek loanwords.

Accommodating foreign grammar is largely just a product of bilinguality. If you have a large number of people speaking both languages, grammar from one slips into the other.

If being colonised multiple times was the relevant factor i suspect berber, armenian, greek, syriac, kurdish, pashtun, maybe a dravidian or siberian language would have english beat easily.