You can buy US Treasuries right now and some CDs and get 4.5+% return while maintaining the flexibility of keeping low cost debt on the properties. 

2) If the lease that the tenant agreed to included them maintaining the front yard/ landscaping - is it reasonable to deduct a 1 time yard clean-up from their security deposit? 

Financial freedom isn't just about replacing your current income; it's about maintaining your desired lifestyle for the rest of your life.

To thrive in real estate investing, you must maintain a firm grip on your finances.

Is it just not my worry what their income is since the voucher covers the rent, or is my concern that if they only have about $950/month then they won’t properly be able to maintain their pets and their standard of living valid?

Small, efficient, if properly maintained will last a long time.  

The new owner bought the house after I purchased my land, and he might not be aware of these encroachments.Is there any risk for me, such as the possibility of acquisitive prescription of a part of my real estate after years, especially if (theoretically) the neighbor incurs considerable expenses for e.g. asphalting his driveway? 

The only way I would be considered liable would be if I was negligent about maintaining the trees.

If experience is the lone qualification, you can align with a good realtor and contractor and maintain far greater control along with benefiting from all of the upside.

Maintenance Costs are Too HighThe cost of maintaining a rental property—especially an older building—can make selling it a viable option.