I'm with herr dunkel on those mergers, but also wouldn't go for radical sweeping changes either. I think, even though a large number of the people are non-native, that many of the clusters and vowel distinctions would effectively remain. Think about it this way. These folks are all going to have children. The accent they speak English with will not be inherited like genes are. Since it is the agreed upon language, why would you not have the native speakers teaching the children English? When they are small, sure they will have quirks in speaking. Think of all the kids under age 4 or 5 that struggle with th sounds or r. My brother had trouble with both into essentially 3rd grade. But if they have an adult teaching them, that person becomes the authority, so to speak, on the subject to them. They will be able to recognize the phonemes in place, and would eventually learn to reproduce them, albeit with small changes likely because of quirks they might develop amongst themselves. Words coming from other languages in, that makes a degree of sense. The kids could use them as slang to hide meaning from adults potentially, or just to sound cool.
The big reason I say this is because, growing up with Greek, I don't speak with a foreign accent (I do have a Boston Accent though). And many of my friends grew up speaking other languages at home too, whether it be Italian, Russian, Spanish, or Chinese. Small accents will develop, but not new dialects over night. Think about poorer regions of Big cities in the US about a century ago. Each neighborhood would have a language from the old country. One would be polish, or italian, &c, but the kids at school would learn English, and because they started young, the accent is minimal. My grandmother talks about this all the time. She grew up in the Quebecois/French region of her city, a few blocks down there were the Polish, then the Italians. She could speak clear English to any of them and get responses, but they still had their family's language. Really, the point I'm getting at is that kids, even if they are forced a new language at school, are VERY good at learning it and using it according to the standard that is in place and taught by teachers.
However, on the other side, if the situation arises that you have something like in India, where some people learn English and then teach it to the children, that is how serious change in the language will take place.
Formerly a vegetable