This is only the second month of Proving Grounds.

You don't need to book the new legos to use them.  I didn't book Varkos or Vallaryn last month, and so far this month, I am using unbooked & ungeared Dorothy & Lion.  

Same.  Its definitely late / end game but certainly doesnt mean you need to be a spender to finish it. 

My first time in proving grounds wasnt a fun experience.  I am still trying to conquer Sintranos and Grim Forst.  They get my focus for now.  

Those two worth a hell of a lot more for your account anyway.  Definitely focus them first. 

Yes.  I patently igrnore it.

Tl;dr:  Yes.

way too strong better ignore it

Yea, tried it twice and I'm just ignoring it now. Way too hard and specifically designed to be as frustrating and unfair as possible. Hard pass.