Sounds like you are having fun already, keep the sailing and the stories coming.
Aluminium tube inside a carbon one is a bad idea, there are loads of broken RS600 and RS800 masts about because they had an ali tube inside which corroded and expanded. Fresh water will not make any difference to the outcome, just make it a little slower.
For the repair I think your plan will be a bit difficult to get right, I have never had much luck with using a bladder.
I would make up a short section of tubing. you want it to have a slightly larger OD than the wing bar's ID. If the laser boom is a tight fit in the current tube, then it would probably an ideal mandrel cover it in melted candle wax so you can get it off.
Once you have the repair tube cut it length ways, and then depending on the size difference either cut a strip out, or sand the slot larger. You want a C section that will close to an O when you push it inside the tube you are repairing.
Once you have everything ready dry fit a few times, then coat the outside of the repair tube in resin and push it in. Job done