so lets examine this

1. life in prison
... A Trump pardon would solve that (disregarding extenuating circumstances). If he flips he gets some prison time without pardon (assuming he provides incriminating evidence of Mr Trump's criminality on something ... so no pardon) but if he actually knows nothing which incriminates Mr Trump, the pardon is back in (maybe).

2. Russian extreme prejudice
... This is more of an assumption than actual death threats. Beside, if he doesn't flip, there is no problem. Suppose he flips and provides evidence against Ukrainians and Russians, has there been a known death threat from any of these folks, or is it just an assumption since it is well known how these gangsters operate?

3. Witsec
... so does he need protection from Mr Trump? He wouldn't hurt anyone physically. Could he order someone hurt? Maybe. From Russians? see above.

4. a 5th of moonshine
... I foresee a pardon of infinite dimensions or forced to respond to what question? I don't remember.

Just some spitballing.

If Manafort actually knew anything, why would Mr Trump give him up?

Speculation: I think SP Mueller convinced Manafort even if Mr Trump gave him a walk, state entities would not and he would therefore still spend a lifetime in prison, however if he ... Now I don't think foreign powers come into play. The accounting books are being investigated, so why would SP Mueller need Manafort to confirm what is already known?

just rambling
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