Sure you lose some momentum on the blade, but the majority of the momentum is from your run, which isn't affected, not the blade itself.
And no, the same move doesn't work for a low-line attack. For low-line, you parry low instead of high, and run down the blade for a backhand slash to the gut or thigh(s) instead of a forehand slash to the head or upper torso.
All attacks are dependent on circumstances to some degree, but one or another variation on "parry the one shot as you close, strike as you pass, be past and out of range before he can respond" is actually fairly common. Any form of what SCAers call "bulldozing" or "an attack pass" is more likely than not to involve parrying a shot as you close. Basically the difference between a "pass" and a "bulldoze" is whether (bulldozing) you are trying to press so close you deny your opponent leverage (he can hit you, but he can't put enough power behind the blow for the hit to matter) or (pass) being back out of range before he can turn and strike.