Of course that's going to be a factor in how believable a witness is. And how they are going to get Jesse to testify? They're going to have give an admitted large scale dealer and murderer immunity, putting his testimony further into question.
This is all hearsay and after the fact. They can't even physically put Walt in a lab, much less near any actual drugs!
Where's the meth?
Lydia already knows the Feds are looking at her. She's not going to just lead them to some f*ckery once she finds out Walt has been pinched. She'll either plot to kill him or dip town with her millions. Nazis only worry is Walt snitching and even then you think the DEA is going to give them a deal on multiple murders to lock up some terminally ill high school science teacher?
It's a catch-22 they can't get the Nazis to roll on Walt w/o getting Walt to roll on the Nazis. If Walt stays quiet they have nothing on the Nazis, who turn have no reason to snitch on Walt. (not that they would anyway)
Why on earth would those people testify about anything? They have NOTHING on them.
DEA - "Some retired high school science teacher, claims he's the head of a large drug cartel and you helped him cook meth in people's houses under cover of your Extermination business."
Vamanos Pest -
"Sounds like a bad tv show buddy."
Outside of maybe Todd, them dudes didn't witness any cooking. They don't even know for sure what was in the cases they were hauling around. That was the beauty of that entire operation.
At the end of the day all Hank has is the money and Jesse.