TTAPI versus TTFIX: Surely this is a question for TT not OQ?
As for using the library without the platform, I'm 99% certain you can e.g. you can leverage the provider and instrument stuff - but you'll probably have to re-write a bunch of code that is currently part of the platform. However...
OpenQuant is anything but "open source" AFAIK.
The naming is clearly a marketing tactic designed to draw people to the product and to jump on the open source bandwagon...
OpenQuant is allegedly a cut down version of the former QuantDeveloper product that I believe was bought by another...