Context:

I want to upgrade my laptop’s total RAM (two 4 GB, one is 2666 other is 2400) and I only have the budget for either two 8 GB RAM or one 16GB RAM.
My questions are:

1. Is two 8GB 2666mhz better than a 4GB 2666mhz + 16GB 2666mhz, and why?
2. Would it better to just have a single 16GB RAM rather a 16GB and 4GB together?
3. Should I consider the CL of my RAM before upgrading?
4. Next time I am upgrading other hardware what should I consider before buying?
5. Also, why can I only use 2GB of RAM? https://imgur.com/9paH259