Search posts on the subject by Shorty. He has a number of posts on hypophosphorous acid made from sodium hypophosphite and HCl. Your point about evaporating much of the water out of the sodium hypophosphite solution before adding the HCl sounds like a wise move to me. You could perhaps even evaporate to dryness, although it isn't necessary.
Keep the heat low after you add the HCl, and rely on the fan for most of the evaporation. The NaCl falls out of a salt-saturated solution. The solution remains salt saturated unless you dilute it with dH20. I had better success using the diluted hypo than I did with hypo I had not diluted. I did not run into shelf life problems with hypo made this way, but I never left it on the shelf for much more than a month anyway.
Heat can degrade the hypo, but not as quickly as you might think. As to the concentration to use: even dilute hypo is very effective for generating HI. You can tell whether you have enough hypo by how quickly the I2 clears after it is added to the flask. With concentrated hypo, the process will be so fast you will have trouble getting a stopper on the flask without losing HI. With more dilute hypo, the process is slower, giving you time to fix your condenser in place before so much gas evolves that you need containment. Even if the solution requires a little heating to produce HI, you have enough hypo in the solution to drive the reaction. If you have any doubt about whether the solution remains strong enough throughout the reaction to recycle the iodine, you can always add a few drops of hypo to the reaction as it proceeds. I found that adding a little I2 -- just a few small crystals -- and watching how fast it dissolved and disappeared-- gave me an idea how well the iodine was being recycled in the reaction. It also indicates that the hypo has oxidized if the I2 does not dissolve readily.
You do not need the hypo at even a 50% concentration for this reaction. If you use concentrated hypo, the reaction will proceed more quickly. You need not hurry it, however.
A reflux for several hours will reward you will a high yield of very good methamphetamine.
You will like this reaction.