MDT TRACON Controller reported an aircraft on approach went below the assigned altitude into a lower MVA; after being in an airborne conflict with another aircraft on approach.

Narrative

Aircraft X was about 8 NM north-east of MDT; direct MDT; descending out of 4;000 ft. MSL for 3;000 ft. MSL. Aircraft X called MDT field in sight approximately over Hershey Medical Center at 3;400 ft. MSL; descending. Aircraft X was cleared for the visual approach to MDT Runway 13 and he was frequency changed to MDT Tower. Aircraft X read back the frequency change.Aircraft X turned to the north-west for a wide (approximately 6-mile) left downwind right after the frequency change to Tower. The aircraft was at 3;000 ft. MSL and indicating about 220 KTS at the time. The expectation was that they would start a base turn no later than abeam ENOLA final approach fix (about 7 NM final). As this is happening; Aircraft Y is approximately 40 - 50 NM north-west of MDT; out of New York Center (ZNY 27 sector) airspace to land MDT. They were indicating about 290 KTS direct HAR VOR. Aircraft Y was initially assigned a descent to 5;000 ft. MSL. Aircraft Y requested to join the localizer to Runway 13 off HAR VOR as a backup for the visual approach to Runway 13. The request was granted but Aircraft X was still outbound on their wide downwind. It quickly became apparent that leaving Aircraft Y on the localizer would create a separation issue between them and Aircraft X on final; so Aircraft Y was issued a 100 heading (approximately 2 NM north-west of HAR VOR) for sequencing and a speed adjustment of 170 kts. Aircraft X was approximately 10 NM north-west of MDT at this point at 2;900 ft. MSL and no indication of a base turn was there so Aircraft Y was issued a right turn to the south-west (approximately 210) at 4;000 for re-sequence. A left turn was not possible at this time because there was an IFR aircraft at 4000 ft. MSL to Aircraft Y's north-west at the time. Moments later Aircraft X transmits on 124.1 that they are responding to a terrain resolution advisory. They were at approximately 2;700 ft. MSL at the time. They were observed climbing to 3;500 ft. MSL; they turned left towards MDT and they were indicating over 200 KTS at the time. Aircraft X was advised that the MVA in their area was 3;000 and that the highest obstruction in their area was at about their 12 o'clock and 1 mile at 1;700 ft. MSL. Aircraft X was asked if they still wanted the visual approach and they said they did. Aircraft X was asked if they still had the field in sight and they said yes so they were re-cleared and frequency changed to MDT Tower. Aircraft Y was crossing over about 8-mile final to the south-west and indicating 170 KTS when Aircraft X took his resolution advisory actions. Separation was observed to drop below 3 NM and 1;000 ft. Aircraft Y was turned further to the northwest (approximately 310 heading) to increase separation and create divergence.