彼を駆り立ててゐたあの執念の原動力は一体何であつたのだらう。

[healer]

Google translates the above as "What is the driving force behind that tenacity that drives him?"

I'm not sure of the grammar structure of 一体何であつたのだらう?
What is 一体 here, one object, one body or the heck?
What is だらう?
Do we always read 何で as なんで, not なにで?

 

[Toritoribe]

What is 一体 here, one object, one body or the heck?

一体 is an adverb there, so it's the last one.

What is だらう?

Historical kana usage of だろう.

Do we always read 何で as なんで, not なにで?

It can be read なにで since it's 何 as "what" + で, not "why", there.