Name translation

I have two questions. First, I try to make a name which means bliss-friend or doom-friend using Sindarin manadh (Quenya: manar) + -ndil. Quenya version of name must be Manardil, if i am not mistaken. Just like Eärnil. But Making Sindarin names are confusing. Only example I could find is Gelennil "Lover of Trees". As I understand last dh is dropping and name becomes 'Manandil' or 'Manannil'. Is -i lenition necessary in this case? It is not plural but Sindarin name Enerdhil (Quenya: Anardil) has i lenition. Second, the name of the planet Venus is given as Elmō in NofME book in Primitive Elvish. Like Thingol's brother's name. How can this word be translated into Sindarin?

There are four valid options: Manannil, Menennil, Manandil, and Menendil. The options with nd would be perceived as more archaic, otherwise they are pretty much all equivalent.

Primitive *Elmō would become elf in Sindarin (pronounced elv), but it's not certain that the name would necessarily descend into Sindarin without some changes along the way.

Thank you.