I pretty much agree with everyting you wrote, except this. I think it's well accepted that you can't increase MHR, although your ability to push hard enough to reach it may increase as you become more fit, giving the illusion that MHR has increased. On the other hand, MHR will go down as one becomes older tho not as fast as the 1 beat per year predicted by the formulas. This decrease can be substantially slowed down if we remain fit, but some decrease from the aging process is inevitable.