The cable that is exposed on the £40 light is under the mount and it is quite open for water to ingress, it is a 4-5mm hole (maybe more, going on memory here) with the 2 inner cores going through it, the issue is not the cable connection to the battery, although this is also not a sealed connector either (the more expinsive versions have a locking sealed connector which is what I am guessing your light has G2EWS), but can be taped if concerned.
As for the vents, on the £40 light, the outer has slots machined into it, and the inner body is a separate piece so again there could be water ingress here especially since some of these slots are on the top/bottom where water will collect and sit depending on the orientation of the mounting.
Rain gets everywhere, really, the smallest hole and it will be in! I had to empty my rims the other week, it must have got in around the valve. Working with water and electronics, I have found vaseline to be a great way to seal things, it keeps the water out, is cheap, and can easily be cleaned off, as opposed to silicon or likewise.
As for the battery pack, put it in a freezer bag and tape the point you pass the cable out and then put it back inside the little mounting pack.