Simulation nodes is in its early stages and I've had some issues with it when trying to render. What fixes the problem in my experience, is making sure you have your settings where you want them and going to the physics tab and clicking "Bake" button, under 'Simulation Nodes' right before you render. 

Yes, however they are roughly 3-5gb of data. They are hosted on google drive, and you will receive a PDF with access to the Google drive folder. Please reach out if you cannot access them.

Google may also ask to download multiple files. I don't know why, but google apparently can split large quantities of files into multiple .zips. It may have you download 2 or 3 separate zip files for each set. Just download them and copy and paste them all into the same folder!

Heavens no. There is so much that can go wrong, or cause issues with 3D projects. Feel free to reach out if you have problems, but even I may not be able to fix things if you have altered the Geometry Nodes setups.

You're on your own.

Blender doesn't recognize Geometry nodes & Simulation nodes as particles. You'll need to create a particle system with roughly the same amount of rain, to bake any Dynamic Paint maps.

Make sure you aren't generating too many particles. Simulation nodes are great but too many particles will still crash your system.

Try baking the simulations via the "Physics" tab to help speed things up.

Simulation nodes sometimes require restarting the simulation. Restart from frame 0 and press play and it should recalculate.

Simulation nodes also allow you to bake the simulation via the "Physics" tab. If you have baked the simulation, this can also affect whether or not your updated settings work.

Yes - https://www.youtube.com/watch?v=Uu7tlXZ1suQ

Absolutely not. 3.6 introduced the Simulation Nodes that this system uses. This does work as expected in 4.0.

No, that would be wonderful, but isn't the case. 

You'll still be heavily involved in the setup process, but this allows you the best level of artistic control. You'll still need to animate the density increasing as the rain starts. For wetness you'll still have to animate the object's materials to animate to a "wet" look.

I shouldn't be telling you this, since it goes against my product's name, but rain is... HARD. It takes a lot of work to get something perfect, but don't be afraid of it! You can do it!

No. There are 3 different setups.

1. A Rain Emitter System - Apply to an object above your scene to cast rain.
2. A 2D Splash System - Apply to a surface the rain will hit.
3. A Droplet Splash System - Apply to a surface the rain will hit.

Yes and No. You can either place it on each object you want the rain to interact with or you will need to create a simplified mesh for the Simple Rain modifier to be applied too.

This can be an easy process, just copy all the objects you want rain to interact with and join them together into one mesh. Then use a highly subdivided mesh placed above and use the shrink map modifier projecting straight down on the "Z" axis to essentially blanket your scene or at least the objects you want the rain interacting with. Apply the shrinkwrap modifier and apply the 2D Splash, & Droplet Sim Geometry Nodes setup to the new shrink-wrapped mesh.

Tutorial here: https://youtu.be/IvxMExjvaBk

Not entirely. It is Using Blender's new simulation nodes system which can be viewed as creating particles, but its not a native blender Particle System.