Re. your questions about SMIC:
My children are in the English track. My daughter thinks that foreigners with good enough Chinese could be in the Chinese track but I don’t know that for sure. Only the English track kids have to abide by the “no Chinese” rule but, I’ve been corrected, they can speak Chinese at lunch. I do believe that there’s a rule that kids in the Chinese track can transfer to the English track when their English is good enough but not v.v. I’m not sure that it would be worth paying tuition for the Chinese track as you can probably find just as good of instruction in one of the better local schools for a cheaper price.
To answer your other questions – My children arrived speaking no Chinese but my husband is fluent which as been a great help with homework.
We are very happy with the school. It has a bit of a missionary population which we were unaware of when we enrolled but it has not been a big issue for us. It is an issue though for some of the teachers who signed on not knowing about this and some seem to leave each year because of their discomfort. I think it’s a bigger issue if you live near campus, which we don’t. There is a lot of homework (2-3 hours/per night) but that seems to be the norm here.
Best of luck with your decision. As I’m sure you know, education is a huge priority for the Chinese so there are many good options.