There are three tool chains available at present for the RL78 controllers.
They are available from GNU(CyberTHOR Studios Ltd.), IAR and Renesas.
The GNU tools are free and without limitations. https://gcc-renesas.com/
The IAR Workbench has a free evaluation version that is full featured for 30 days then restricts the object size to 16K bytes.
Without requesting a quote it's difficult to get hard numbers for an IAR license but from what pops up from web searches are amounts in the US$3000 range.
The Renesas CCRL compiler has a free version that is object size limited. The annual license subscription costs about US$300, the perpetual license is about US$1200.
Over the past 2 weeks I have been using the GNU tool chain from CyberTHOR. From looking at the object code output it appears to have a good code generator.
The RL78 has been around for some time but this is my first deep dive in to the architecture. It may just be me but it has been a lot of trouble to get the e2 Studio IDE and GNU tool chain installed on a Windows system and working reliably.
My company has a product developed by a third party that uses an RL78/G14 class controller. My task is to support the production testing of the product. To that end we need to develop code for the RL78 to do fundamental hardware level tests at the PCB assembly level.
I have digressed a bit so to get to your points:
1) For years Renesas has supported GNU tools for most of their controllers. CyberTHOR just seems to be the current partner. The KPIT GNU tool chain for the RL78 seems to be in archive mode.
2) As a Windows user I don't rebuild the tool chains from source so have not needed paid support from the distribution provider. As for using the compiler the online GNU documentation and the Renesas users forum are the support that has been helpful for me.
3) My opinion is that the GCC compiler is equivalent to CCRL. The Renesas CCRL compiler seems to require using the IDE (e2 Studio, or CS+ previously know as CubeSuite Plus). The e2 Studio IDE is Eclipse based and is a real learning experience for me. Renesas has a driver source code generator in the IDE. This works but is tricky to use. Renesas has other driver source code generators, Applilet3 and AP4 for RL78, that you may find helpful.
4) I have not yet observed the GNU tools output excessively clumsy code for the RL78 but I still have a lot to learn.