A TRACON Controller reported an aircraft being vectored for a Visual Approach deviated from its assigned course and altitude resulting in them flying below the Minimum Vectoring Altitude and into conflict with VFR traffic.

Narrative

Working entire final. Aircraft Y on east downwind going to Runway 35L in front of Aircraft Z doing RNAV Z to Runway 35R. Was missing pointed out traffic at 080 going into CFO and other VFR [traffic] in south vicinity of CFO. Aircraft X was on Heading 325 [degrees] to join the Runway 35L localizer. I was waiting to issue Visual Approach clearance until I had based the Aircraft Y as that was the part of the sequence that needed to be more timely to keep the Aircraft Y in front of the Aircraft Z. I issued an approach clearance to Aircraft X and observed the aircraft altitude 9;100 ft. I knew that I had issued the aircraft 10;000 ft and wanted to make sure the pilot had the correct altimeter given it appeared the aircraft was 1;000 ft low. Pilot reported 9;100 ft and I advised pilot I had issued 10;000 ft. He advised that he read back 8;000 ft. It was a moot point at this time. Moments later the conflict alert went off for a VFR [traffic] that was just ahead of Aircraft X. I issued a traffic alert and suggested Aircraft X stop descent as Aircraft X altitude was 8;100 ft and the VFR [traffic] indicated 7;600 ft. No response from pilot. I issued the traffic alert two more times. Then Aircraft X was now at 7;100 ft and receiving a low altitude alert so I issued a low altitude alert; then asked pilot to verify you have all the terrain in sight. He did. Then Aircraft X appeared to be intercepting the Runway 35R localizer instead of Runway 35L. I advised pilot of this and instructed him to turn left to 330 heading to join the correct localizer as the Aircraft Z was on the Runway 35R localizer. Best recommendation would be to lower the Class Bravo Airspace around Denver airport to 7;000 ft. Way too many VFR [traffic] traverse below the Class B which is underneath our four final approach courses when landing North. It is an incredibly dangerous spot for them to be and because of the proximity to APA airport there is VFR [traffic] everywhere.