Heya and welcome here.

I unfortunately do not know if installing Ndless and playing ASM/C games can void your warranty, as people played ASM stuff on their TI-84 plus, crashed their calcs many times in the process, and when it broke, they had no problem exchanging it in stores. If your warranty requires you to ship the calc to Texas Instruments, then I am unsure, though. It would really have to be completely bricked, which has yet to happen. I think the most dangerous file so far is Nover.

Sadly there are no Pokémon remake for the TI-Nspire, but there is a Game Boy Color emulator called gbc4nspire that allows you to play all Game Boy and Game Boy color Pokémon releases. We cannot give links to the ROM files, though, as it would be against the forum rules. Google searches will most likely give you decent results. The emulator is located in the downloads section, along with Ndless 3.1.

Downloading and playing games on the CX CAS is safe, and has not been demonstrated to void the warranty
Some of TI's own OS upgrades (OS 3.0.1.1753 + boot2 3.x) for the older Clickpad and Touchpad models brick calculators (*); nothing of that magnitude was reported for any Ndless version, over nearly two years.

(*): after upgrading to OS 3.0.1.1753 without removing the enclosed boot2 3.x upgrade through TNOC ( http://tiplanet.org/forum/archives_voir.php?id=1922 ), hundreds (at least) of Nspire Clickpad / Touchpad users found, one or two reboots later, that their calculator wouldn't boot anymore, and failed to communicate with a computer through USB... IOW, their calculator could not be fixed through usual means. Calculators in that state need boot2 reflashing through the serial port, and can be done by third parties through additional ~$15-$20 hardware.

THat said, I think if an Ndless program permanently brick the calc and they discover Ndless is installed, the warranty would be void, right?

Well, if it were, then the usage terms of the calculator would be pretty obscene
Users are free to use the hardware they own the way they see fit - even if the manufacturer doesn't like it. And besides, it's unlikely that TI's calculator fixing service checks each and every calculator for Ndless or whatever else

Ndless is, in common terms, the Nspire's jailbreak. TI made the Nspire a platform closed to native code (C/ASM) programming (which is a major shame because it's the most powerful calculator on the market), and has stuck to this silly stance for five years; but Ndless forces it open, and provides functionality that other native code programs can build upon.