I can see him on the multiple cameras I have in the home.

There was even a hot cup of coffee on the dining table.So we bought some Arlo security cameras. 

Other thoughts are perimeter fencing, and possible surveillance cameras.

Have you considered installing CCTV cameras at your property?

While living in the small unit downstairs, we went through some noisy tenants who kept us up all night and racked up utility bills, to tenants who never got along, had three times the number of cats stated on the lease, to one of them spying on each other with a hidden camera!

Do they set up surveillance in the Home Depot parking lot and follow people home and ask to see their permit?

Newer homes in my area have overhead sewers, which are not buried in the ground, until you get the outside.Sewer companies have sewer cameras.  

agreed with the above. if it is within your budget, you can also set up cameras on your property pointing towards the alley. public defecation is a crime and violators can be listed as sex offenders in some areas.

I know much of the issue is from a camera problem. 

Just leave a note saying you entered to fix or service what was needed..if they won't let you in I guess you call water dept and have water shut off at the curb if it's water related.. call gas and have gas shut off at outside of bldg if it's gas related, notify police your building is unsecured and you have attempted to secure but have been barred access and need escort.. and you file eviction emergency action for cause I always have phone camera ready..the he said , she said on this will be a ongoing situation.. get them both out.. mutual lease termination if possible.. just document noise infractions,, and damages,, police calls are not infraction if they call themselves.. if neighbor call police get copy of report.. if you call its a unsubstantiated report/ shows bias and cant' be used as reason for eviction. better if tenant or neighbor calls.One way or the other they will get out.. damages will happen and better to safe guard yourself then to get between 2 crazy people.. the cops out there will tell you the most violent unpredictable calls are domestics.. and people get hurt, killed,, don't let it be you.