If you want a new 686 get one, and don't worry about the lock. I also have several S&W N frames that I use for work. My 627 Pro and 629 both have the lock. Both have several hundred rounds through them, with the 629 having mostly full power 44 magnums. Neither has ever given the slightest indication of an unintentional lockup. I'd only worry about the lock on the extreme lightweight, heavy caliber guns like the scandium 44 mags and such. I think a dead primer or other ammunition failure would be a more likely scenario, than an unintentional lockup. I've seen a couple examples of that happening, never a lock problem.

It's not illegal to deactivate the lock, at least in my part of the country. (May be in one of those north eastern states.) I don't think you'd have any issues if you deactivate the lock and end up using the gun in a lawful shooting. As long as everything else about the shooting is legit, it shouldn't be an issue. (Not intended as legal advice, just my personal opinion.) But lawyers being lawyers, if you have "deep pockets", they could go fishing in that direction. Who can say for sure one way or the other, I know I can't. I wouldn't worry about buying or using a S&W 686 with the lock though.

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Tom
"Panem et Circenses"