Wishbone, to quiet the decocker, you can retract the slide 1/2", then release pressure on the frontstrap-squeeze cocker. That's as quiet as it can be made to be. (Perhaps you already knew that.)
I've never had any trouble putting the slide back on the frame, but the P7M8 was my first handgun, and I've been field-stripping the pistol for 15 years. You just fit the front of the recoil spring, which is stabilized by being held in place by the barrel (which it surrounds) over the barrel hole in the inner front of the slide, while allowing the gas piston to hang down, and sort of "finesse" it all back together by balancing the forces as you pull backward. One hand holds the frame, the other hand holds the slide. It's a sort of "learned skill", but one which becomes automatic after being done hundreds of times, and the gas piston will slip right in where it's supposed to, even if sometimes a little wiggling is necessary. Never takes more than 5 seconds from picking up slide & frame (with spring already on barrel) to reassembly. Maybe your P7M8 is new, and internal edges are all a bit sharp yet?
Jeff OTMG. your estimate of "thousands" of these guns coming into the country is overstated, I think. According to H&K, there were "several hundred" German police trade in guns up for refurbishing and resale. There aren't so many of them that you'll see new ones going for less than $900, and the "trade ins" have been going for as little as $525, or as much as $700 in the advertisements I've seen. (It does seem that the $700 figure is a bit high for a gun that's been cosmetically damaged, even if the guns are only "slightly" used.) The introduction of several hundred extra P7s into the USA isn't going to devalue the gun much, and if such brings it more within the price range of some who otherwise wouldn't have considered purchasing one because of the cost, how much the better! I'd encourage anyone seriously considering the acquisition of a P7M8 to go ahead and obtain one of the police trade ins before they're all snapped up, if you can find one for a good (< $650) price. And, although some of the guns are being sold without warranty, others of these same police trade ins are fully warranted by H&K; the one I bought, for instance, is. I haven't heard any explanation as to why some are provided with warranty and others are not. While these are extremely reliable pistols, and I've never had anything on mine break or require replacement after uncounted thousands (> 10K, anyway) of rounds, there's still no reason to buy one from someone who tells you "there's no warranty" unless you can get a better deal (say, ~$500) as a result.
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"Potius sero quam nunquam."