The original ready room model was made by Sternbach and Probert.IIRC, the Reliant kit wasn't released until 1994ish.
Yes, they used Gundam kit parts for greeblies. I think Greg Jein made it, but I may be wrong. Just plain silly, IMHO, but I guess even in the 80s they didn't think it would be so closely examined.
Personally I like the greebled look for TMP-era designs, makes them more utilitarian-looking and thus the evolution to the more elegant TNG-era designs more apparent.
Plus it had a real-world purpose: to hide access panels for lights and supports. The refit Constitution was a very smooth looking model, and ILM hated working with it once they received it for TWoK, because it had no access panels so it needed an almost full disassembly to fix simple stuff. It was this very fact that made the model impossible to be used for a TV schedule, which they realized for 'The Battle' and required the larger Constellation model to be built after the episode was already filmed.