Are there plans to update for 1.4...
or for RimWorld 1.5?

Last two are in the same pool as exotic goods. So obtainable through quests, ancient dangers, and some traders.

how do you get the last two weapons they aren't appearing under any research

No I made it so only your colonists should have access to the weapons. Creating units that use the weapons that do not have neural heat gain might be something I'll look into, no promises right now though. Giving ai units the weapons that have neural heat gain seems like a larger task than I'm willing to take on, and I worry might cause a lot of problems if implemented.

I love the idea of this mod. Do empire faction members spawn with these?

This mod never received the attention it deserves as a proper hidden gem, these all seem like Wondertainment scps and they're so cool, despite the lack of context images of the items ingame, they are really well done, I was impressed by the quality.
There are plenty of room for expansion though, it kinda works with Infusion 2 which adds more benefits to already weird weapons, a patch to add more inclusive mutations with infusion, plus something to add a bridge for the many psychic mods and the upcoming VFE Ancients would be awesome if you're willing.
Thank you for your hard work.