I can't comment on the two listed above too much, but i will comment on the LB Plat. I recently purchased the Lawn Boy 10795, which is similar to the 797, but without the Blade brake. I love it. I was down to that and the toro, and the it was a close call, but went with the LB due to the reputation of the Honda engine and I liked the bagging system. I looked at teh HRX honda at Home Depot, thought it was nice, but was turned off by the drive system. Also, not 100% on what it means, but the Platinum LB has a sealed drive system that is upgraded from the lower level LBs.

I removed the upper blade for mulching, and it went a long way toward 'fixing' the problem. Definitely clean the deck as well as you can after such use. You shouldn't have to sharpen the blade(s) more than three or four times a cutting season. The great thing about the Xenoy (or Nexite) decks is there can be no corrosion from the residual build-up, and it can't really adhere like it will with a metal deck. I buy a pack of the 8" tapered wood shims at the hardware store for scraping the crud out without damaging the deck.

When you clutch the blade, the engine loses the blade's flywheel effect at the same time the governor lets the throttle close a little while maintaining the governor RPM setting, probably around 3000 RPM. You might expect the engine to sound different at this time. I think it may have a sticky valve, but its not really a problem since the engine responds to the governor and runs properly under a load. You could try using a mild solvent in the oil and gas to see if that helps. Get some Marvel Mystery Oil at Wal Mart or any auto parts/supply. Put 1 oz in the gas tank and 1 oz in the oil. The measure doesn't have to be precise - a little more won't hurt anything. Give it about two or three cuttings to see if it makes any difference.

Whoa don't put a lock washer on it. I believe you have a bent crank shaft that is causing the screw to back out because there is now an extra vibration while it runs. Anytime you hit something and the blade stops the engine it's not good. You should check the crank for bending by fliping the unit over and slowly pulling the rope while you look at the crank (with the spark plug wire pulled off). If it looks at all bent you need to repair it. I've seen this so many times where people let it go and the vibration causes many other things to break prematurely.