Since it seems like you're speculating, I'll join in. If anyone knows, I will withdraw all such speculation.
On the first point, I'd be surprised to find sealing the tube is a negative, since the plume continues to fill the volume behind the rocket as said volume increases.
But if the tube is sealed at the bottom end and not completely filled by the rocket the flame will precede the rocket out the front; we've all seen pictures of that, I'm sure. So I would think the finish would be badly scorched. One might consider using a sabot to fill the tube and protect the rocket until it emerges. Which would increase the pressure behind the rocket and any gun barrel effect, as well as protect the paint.
Regarding the second, I think you've got it right, 99%. The difference with a rocket that's on a guide and also in free air is that the fins or GDS (or whatever else) begin operating, as you said, before the end of the guide. The two means of assuring straight motion overlap. With a tube launch there is a hand off from one to the other with no overlap. Is there a moment after emerging from the tube that, even though there's enough speed for the fins or whatever to work, the necessary air flow geometry is not yet established? A hand-off transient? And if so, does it last long enough to do any mischief? (If it's scant milliseconds then simple inertial will keep the rocket going straight enough.)