This is why US should never allow blanket pardons or pardons before a conviction.

Not sure if this is really a great example to make your point, considering Maxwell was convicted already, and Trump wouldn't need to issue a blanket pardon to get her out of prison in exchange for her testimony.

The real issue here is that Trump knows there's no consequences if he does pardon her. He wouldn't get removed from office over it, and I doubt the House would even bring articles of impeachment. The Supreme Court has conceded him all the power, and Congress is too spinless to hold him accountable for anything.