I have an android streaming client application which uses Java and cpp code. I want to port my application to Roku. I understand that Roku recommends only BrightScript for apps development.
1) According to this link (C/C++ development in Roku), gaming application can be written in cpp using marmalade and compiled for Roku.
2) But this link (How feasible is Roku app development with Marmalade?) clearly says marmalade can not be used for streaming application.
3) My questions are (A) Is it possible to compile cpp for for roku?
(B) Where can I find the toolchain for building?
(C) If UI is developed using BrightScript, how can I call cpp functions from it?
You can't. Not independently from Roku Inc anyway - you will need a very close collaboration from them, which means you should be able to persuade their business development/partners department you are worth time and money. In other words, your company better be a household name.
You cannot "find" the C toolchain, it may only be bestowed to you by the Co on individual basis, after approval and under NDA.
If you don't have special relationship (project/contract) with Roku Inc, for non-games you can only develop in BrightScript. It's a mandate, not recommendation.
Specifically for your case, best option seems to be to use the already existing streaming formats/components the Roku API already provides. BrightScript is a script language, not fast enough to write a codec.