Coming along nicely Dave. In answer to your questions -
1. Yes, just bond to the outside of the hull. If you have any concerns at all about strength, put a couple of pins through the hull and into the skeg, but a decent glue will normally suffice.
2. That construction sounds pretty robust. I assume you will use a polyurethane or epoxy adhesive to glue the skeg to the prop shaft tube, and these can fill most of the gap that might be there between tube and skeg. Or you can be a bit clever and groove the edges of the two parts of the skeg so they fit closely around the tube. That isn't as hard as it sounds, 10 minutes with a rat tail file will get you pretty close to a good fit. When you glue the cheek pieces on, I suggest wood glue for the majority but a strip of epoxy (or whatever) along the line of the prop tube.
3. I normally just use a wood filler from the DIY shop. I agree polyfilla probably isn't the best, but any proprietary wood filler will do just fine.
4. I'm not sure I can recommend a sanding sealer - the container I have is as old as the hills (and I can't remember what it is without going out to the man cave), and is the only one I have ever used, so can't offer any comparison. But don't overthink it - I suspect whatever you find in your local shop will do just fine.
5. I haven't got an oiler on any of my boats, but I do use waterproof grease in the shaft tube, and I renew it every year, (or when I remember
), and have had no problem with either leaks or excessive wear. You don't need to fill the whole tube, I use a quick squirt of something like
this in the wet end of the tube before inserting the prop shaft. Others I know religiously put oilers on all their boats, so I guess you can take your pick.
Hope that helps you on your way. Enjoy your build!
Greg