I think you are missing a couple of points.
I don’t think Trump cares about passing any legislation. Consequently I don’t think he cares about shields. I think he cares about his own ego and the attention he gets.
Secondly, The problem we see at the moment is a political crisis which is easy to underestimate. The GOP knows they are the most unpopular force in American politics, *way* behind Trump (and it is not close). So they won’t impeach because they are afraid of the voters. The Democrats want to impeach for the wrong reasons. Comey’s testimony being one which lacks smoking guns is unlikely to be particularly problematic for Trump.
This isn’t to defend Trump. He’s pretty indefensible, but so is everyone in Washington these days.