Q: Why is there only one SODIMM in Fitlet2? Doesn't it affect the performance?

Apollo Lake architecture supports dual channel, but there is no performance penalty in using single channel (unlike Core CPUs, where dual channel improves performance).
There is also no RAM capacity restriction. In fact, fitlet2 supports 16 GB RAM rather than 8 GB max as stated for Apollo Lake.

Q: Does Fitlet2 support ECC RAM?

Fitlet2 does not support ECC RAM. 

DDR3 SODIMM ECC RAM requires a different pinout than non-ECC RAM.

Q: Why DDR4 has not been implemented for this new Product?

LPDDR4 (RAM) is not SODIMM component + cannot assure 64 bits systems.  

The only SODIMM component possible for Fitlet2 is DDR3L.

Q: Why TGPC and CompuLab have 16GB as option while Intel Datasheets show up to 8GB RAM?

Don’t be surprised to find 16GB. Intel has introduced Atom and Celeron Apollo Lake Processors with a maximum memory capacity of 8GB but we have successfully tested in the lab with 16GB ram and will offer this option to you. This is very impressive for a form factor this small, and against all other PCs with x7 E3950 or J3455 in the market, like the NUC version, which are commercialised with up to 8GB.