Studying and Teaching

Are the writings of Rabbi Ramchal reliable for Torah study? I am asking because I heard there are some controversy about his writings?

"My spirit shall not abide in man forever, for that he also is flesh; therefore shall his days be a hundred and twenty years."(Genesis 6:3) The verse seems very difficult for me to understand. Would you explain it to me understand it easily?

If a non-Jew was makdish a behaima tehorah for bedek habayis, would it automatically ascend to have kedushas korban as a Jew's would? And if so, would it lose its din of meilah?

"And I will give over the Egyptians into the hand of a cruel lord; and a fierce king shall rule over them, saith the Lord, the LORD of hosts." (Isaiah 19:4). In this verse who is this king?

Parasha Chukat Numbers 19:1-22 discusses the Red Heifer and purification. Numbers 20:1 The entire congregation of the children of Israel arrived at the desert of Zin in the first month, and the people settled in Kadesh. Miriam died there and was buried there. How is verse 20:1 interpreted to mean "AFTER FORTY YEARS of journeying through the desert" they arrived at Zin? There is no mention of years.    

Parasha CHUKAT NUMBERS 19:11-12 The person who touches a corpse is unclean for 7 days. Therefore, cleansing on the 7th day makes sense to me. Do the days represent the number of days after touching the corpse or the days of the week (3rd day would be Tuesday, 7th day would be Saturday)? Main Question: What is the significance of the third day? Thanks.

"The people stood far off, while Moses drew near to the a thick darkness where God was." (Shemot 20:21). My question is: Does God resides in thick darkness?