Upgrade storage Latitude e7450

Hello, 

I've been looking at the requirements to upgrade and install a second disk on my Latitude e7450, and I am a bit confused with the procedure and disk specs. Currently, I have a SK hynix SC210 mSATA 256GB (factory).  

I talked to a Dell representative and he told me I can upgrade the SK Hynix to a Dell M.2 PCIe NVME Class 40 2280 SSD - 1TB (although I am looking for alternatives as this one seems to be a bit pricey) and install a second 2.5'' SATA disk. But I don't understand the procedure as all I have found so far seems to replace the M2 caddy to fit the 2.5'' SATA disk and install the current mSATA in the WWAN slot.   

So, my questions are: 

1. What are the specs to replace the factory SK hynix SC210 mSATA 256GB disk (mSATA/M2, PCIe/NVME?)
2. Can I install a second 2.5'' SATA disk as well? 
3. Can I install the factory SK hynix SC210 mSATA 256GB in the WWAN slot? 

Thanks a lot for your help, cheers,