I think your confusion might just come from not understanding the idioms in the sentence, so I'll start by paraphrasing it.
There isn't an entomologist in the whole world who wouldn't give all he has to be in my shoes today
Since there is a double negative, we'll go ahead and reverse it. Hopefully this makes the sentence less confusing already.
Any entomologist would give all he has to be in my shoes today.
2 of the phrases left are give all he has and to be in my shoes.
You seem to be trying to split it into would give all and [if/when] he has to be in my shoes, and you are reading "has to" as "must". This is incorrect in context.
To give all he has is to sacrifice anything he owns. To be in my shoes means to be experiencing what the author is experiencing (catching the rare insect)
So the whole sentence simplified is:
Any entomologist would love to be in my situation
It should be clear there is no room for if or when in that sentence.