If IMS is your target, at the moment you may also consider DMS - Discovery Montessori School. It is a great school and the mandarin should be as good as IMS for their Central Campus (DB campus mandarin not as strong due to the bigger expat population). The only drawback is the lack of a Primary school, but then many kids leave IMS after kindergarten, so if you wish to put your child in IMS primary, you may try to apply again later, and being in DMS should keep the transition smooth and easy. However, note that the playground language of both IMS & DMS are English, so if you ultimate target is to have your child in a Chinese immersion kindergarten and then primary school, then both schools are not very suitable. (They are bilingual schools, not immersion) If you want Montessori in Chinese, there's a school on the Kowloon side called Best Concept. Their website is a bit primitive and they seldom answer the phone, but from what i heard and saw from my friend's kid going there, Best Concept is wonderful, even more pure Montessori then IMS & it runs mainly in Mandarin.
St. Stephen's Girls and KCS are very popular too, but never hurts to try. Most local schools do interview in Canto, so you really really need to brush up on her Chinese if you are planning try for local prestigious. For prestigious local primary schools, by P1 interview season Hong Kong parents usually have ensured their children know at least 1 to 2 instruments + do 1 competitive sport + have certificates from drawing classes + win a few speech competitions in mandarin -- not that you have to follow the crazy trend but that's the reality. There are local schools that cater for non native Chinese speakers. Our friend Howard here is the expert, and if he is around to see this thread he should be able to give you some pointers (or just search past threads and find his posts).
At this point i'd say you just try to put her in a quality neighbourhood kindy for September to brush up her Chinese. Again, as long as you are just going for any kindy, there are sufficient spots at least in the PM classes. Since your child is already a native English speaker, going back to Int'l School should not be a problem as long as you register the good ones by this September/October for K2/Reception next year. Note that LS interview happens around Nov/Dec while IS interview happens around Jan/Feb/March, but both must register around Sept if not earlier. All the best to you in school hunting!