L30 Controller described MVA and separation event when a VGT departure failed to follow the SID track and maintain altitude; pilot claimed severe turbulence encounters prevented compliance.

Narrative

Aircraft X departed North Las Vegas (VGT) airport. The aircraft was released on the North Town Two departure procedure with an assigned initial altitude 7;000 FT. At the same time; I had accepted a point-out; from Nellis Approach; for an aircraft; north of VGT; landing VGT. Nellis advised my Traffic Management Unit they would stop their pointed-out aircraft above my departure. Upon departure; Aircraft X checked in and briefly tagged-up with the aircraft RADAR information; the information immediately dropped off the scope. I instructed; Aircraft X squawk assigned code and ident. Aircraft X idented the correct code; but did not tag up. I observed the correct beacon code and RADAR identified the aircraft and assigned an altitude of 7;000 FT. I noticed Aircraft X climbing through the assigned altitude. I tried to verify the assigned altitude of 7;000 FT and on the North Town Two Departure. While this was happening; I also notice Aircraft X continuing on a northbound heading. Aircraft X should have been turning to the east to intercept the LAS 320 radial inbound to LAS. I wanted to keep Aircraft X climbing in order to get above the point-out traffic from the north. I assigned a climb to FL190 and a right hand turn to 130. At this point; Aircraft X started turning to the west and descending. I wasn't sure why the aircraft was doing this; I could only assume the pilot was trying to rejoin the North Town Two Departure. Since the pilot started descending again; this time below 7;000 FT; I once again assigned the pilot an altitude of 7;000 FT; and asked to verify that he was flying the North Town Two. That was the only conclusion I could factor; since the pilot didn't take my last assigned heading or altitude. After assigning 7;000 FT; I observed the target climbing through the assigned altitude. I issued traffic to Aircraft X to the north as well as terrain to the west. I could no longer issue a climb for the terrain because the traffic at 8;000 FT. Aircraft X was instructed to turn immediately to the left heading 130 and maintain 7;000 FT. The pilot advised he was in severe turbulence and having trouble maintaining altitude. The pilot was able to eventually level the plane and take the assigned heading. During this incident Aircraft X did briefly break separation with the point-out traffic and also went below the minimum vectoring altitude. I issued control instructions to avoid both of these occurrences. The pilot not flying the assigned departure procedure; headings or maintaining the assigned altitudes restricted my ability to avoid these circumstances. Recommendation; insure pilots are familiar with the North Town Two Departure.