1. It is considered common practise for investigators to hold their cards close to their chests, choosing when to release snippets publicly to control it's impact.
This.
And especially true of someone who's big mouth got him in so much trouble during the election. He is naturally gun shy.
2. Comey isn't investigating Trump per se, In spite of the presidents repeated attempts to becoming the centre of attention. Comey is investigating Flynn et al. Some people wonder if Trump's a little too close for comfort to Flynn, what with Trump firing Comey and Sally Yates because of their views on Flynn.
Exactly. President Trump could have sailed over this whole mess. (Unless there is a smoking gun somewhere.) If he had (and this is a common thread) stayed mute on the thing he would not be in the trouble he is in now.
They have him on obstruction of justice right now. A first year law student can see that. His comments to the Russians confirm it. His best hope is that the democrats would rather have him than VP Pence. That very well may be the calculus now. Some moderate Republicans up for election are probably right there with them.
Flynn is going down. Big time. Whether he is visible to play the Ollie North role remains to be seen. No doubt a couple of other public figures will take a fall as well. The ones to watch are Jarrod and Ivanka.