There are three different G3 15 models -- the 3500, 3590 and 5525.  You'll need to supply the exact model (look on the setup screen, F2 at powerup) for a definitive answer.  The newer models can in some case take  two M.2 drives;  older ones take one M.2 and one 2.5" drive.

In most if not all of these, the keyboard itself cannot be replaced short of replacing the entire palmrest assembly, to which it is permanently attached.  The biggest issue isn't usually the cost but the fact that the entire system has to be torn down because everything inside from the display to the system board is attached to the assembly.  Again, a definitive answer requires the full model number.

parts-people.com is one source of parts but if it's a newer system, you may have to call Dell to order.

Yes that SSD is compatible. 

But for second SSD availability please visit support.dell.com enter your service tag 

go to documetation > Specification: Under storage section you can find, all supported SSD/HDD confirgurations

So, I'm mildly confused from what I see on the page. Can I add up to three different types of storage?

Dell G3 3590 Setup and Specifications | Dell US

Your model support one m.2/nvme SSD which it already came with 

and a secondary 2.5-inch drive which is optional. 

Your HP SSD will replace the curent SSD which is installed on the machine. 

If you wanna add a second SSD you will need one such SSD TBCDAs well as hard drive mounting bracket along with it. 

How To Upgrade The Dell G3 15 To A 2.5" SSD - YouTube

Mine is the 3590.

You can use either Optane or NVMe M.2, plus one 2.5 SATA" drive -- total two drives.  If you did not order the system with a 2.5" drive you will need to order the bracket, cable and screws from Dell or a Dell parts seller to mount a 2.5" SATA drive.

The keyboard-palmrest-toucpad assembly is this:

https://www.ebay.com/itm/384457002546?epid=20037441267&hash=item59836c4e32:g:uLAAAOSwvrthcIXT&amdata=enc%3AAQAHAAAA8BBm9pdjjQZi8BMlVHZfIBOsM4tGOhur5El67wUo3BrxNlcL0HVBwU7Jcqm12r8VhheKHjxc9OSNFPcTb5l8iSXA58TYrBRPPuF8E0odTypiVoRPKZIdhFCE6z93hF6WPZtVlqigRAcNq8FIPFhumHQSn6Ekgu58O9CGXf8UcPj6KRlrlvjcuCsMLp3TpjDeXGv685Eqcp43%2BgCQQuWDiWcw2W%2FN66d5CS%2BRvFS5SA7wXn4UtHY5rBMpn6Mxl1saiH2Xh5Ga5TLX2e4%2FAPYXjYwVMpQUfm2JXHewxTIfrVUOg2TdPxrGgeDY9GeNoP7xng%3D%3D%7Ctkp%3ABFBMtKj3hKpg

No you can not 

Machine has 2 storage slots, one m.2 and one 2.5 inch \

M.2 slot supports both smaller (2230 size) and regular (2280) Nvme SSDs 

2.5 inch slot supports both regular hard drive and solid state drive. 

they can be utilized in multiple ways. 

1. Only one NVME SSD (this is what you have)

2. One NVME SSD and One 2.5 Inch drive ( this is what's best ) 

3. One 2.5 Inch drive with a Optane memory  (Won't recommend) 

I found this for the keyboard, is this the same thing?

Replacement for Dell G Series G3 15 3590 Laptop Upper Case Palmrest Keyboard Backlit Touchpad Assembly Part 00JP6X 0JP6X : Electronics (amazon.com)

Would any of these be acceptable solutions?

HP EX950 M.2 2TB PCIe 3.1 x4 NVMe 3D TLC NAND Internal Solid State Drive (SSD) 5MS24AA#ABC : Electronics (amazon.com)

Amazon.com: HP FX900 1TB NVMe GEN 4 Internal SSD, PCIe 4.0 16 Gb/s, M.2 2280, 3D TLC NAND PC Solid State Drive Up to 5000 MB/s - 57S53AA#ABB : Electronics