Out of four sunroof sets I have worked on, I've never seen a blocked drain tube yet.
Any water in the tray that gets past the glass seal can leak out under the screw heads that hold the sunfoor cassette together.
The most likely place for water to get into the car directly is between the sunroof and the roof itself, the seal is a miserable affair.
You can also get water in through the roof bar mountings.
Sikaflex 221 Black to stick the sunroof top down to the roof. Don't just squirt it in the gap, take the roof out and clean it up, put a 6-8mm bead of sealant in the middle of the flange all the way round and carefully lower it into place.
Headlining takes an hour or so to get out, then it is all there to see. Be gentle getting the drain pipes off the plastic corner pieces, they are not available as a spare part if you break one.
Peter