Based on your description, my advice would be to fix one thing at a time. It sounds as if the air leak around the bellows frame is the most serious thing, so concentrate on that for the time being. Renewing the bellows gasket is a straightforward job - just a bit of care needed.
1. Pull out the bellows pins (use pliers and a straight vertical pull) which will enable you to separate the bellows from the end frames (do one end at a time so you don't get the orientation muddled).
2. Use a blunt knife or similar to scrape off the old bellows gasket. Take care not to gouge any grooves or nicks in the wood of the bellows frames. Ensure the exposed surface is clean and free from old adhesive gunge.
3. Replace the gasket with the proper self-adhesive neoprene gasket available from Charlie Marshall (he ships overseas).
See here: http://www.cgmmusical.co.uk/CGM_Musical_Services/Bellows_Gasket.html
4. Cut a length of gasket long enough to go all the way round the bellows frame with about an inch extra for final trimming/overlap.
5. Start halfway along one of the short sides and gradually press the gasket gently into place, removing the backing paper progressively as you go. The important thing is not to stretch the gasket, especially when going round the corners. Don't use any additional tension, just ease the gasket into place - it will flex and bend around the corners.
6. Finish off by making a diagonal butt joint with your starting point. Lay the one end of the gasket directly over the starting end and carefully cut down through both layers with a sharp blade, at a 45° angle to the length of the gasket. Remove the excess pieces with fingers or tweezers.
7. Do the other end in a similar way, then reassemble everything. Reinsert the bellows pins with a straight push using a piece of scrap wood or similar to protect your fingers/thumbs. You will need to compress the ends against the gasket a little in order for the pins to find their original holes. Hammering the pins shouldn't be necessary. If the pin won't go back in with a push, you probably haven't got the holes aligned properly.
It is just possible that the air leak which you have described is due to the bellows frame coming loose from the last cardboard fold in the bellows itself. I've had to repair one HA114 with this fault. In this case, remove all the bellows pins and separate the bellows form the instrument. Run a little PVA glue in the gap between the bellows card and the bellows frame and clamp firmly (but don't distort the frame) or better - lay on a flat surface with a a few heavy books on top and leave for a few hours until the glue is set.
It's still worth replacing the bellows gasket if it is old and compressed.