1. If someone is using part of my property as a path for 20 plus years does that become a public path. If not how do I stop them from using it and errect my fence.
It is illegal, especially f they already have a path way for accessing their property, encroaching your property for accessing their property is absolutely illegal, both criminal and civil cases are maintainable.
2. What about the open land where they are tieing a cow in the day and have planted some vegetables and bushes. Can that be stopped and land taken back by me.
Yes, it being yr property you may ask them to vacate and take back the possession, if they refuse, you may file a suit for mandatory injunction and also can seek their ejection and vacant possession.
3. What about the tin tempory bathroom and the septic tank. What action can be taken and how can they be vacated or is it better to leave it to them.
Get back all your property, should not show them any sympathy towards an inch of occupied property too because it will pave way for larger encroachment.
4. They are also throwing dirty water etc in a section of the land. How can that be stopped.
Besides suit for mandatory injunction, you may lodge a complaint with the local municipal authorities too.
5. Lastly how to fence up my property with the minimal issues and cases, as a case can go on for years and years in our current legal system. These are very aggressive type of people/ family.
You can engage a government surveyor, get your property surveyed and encompass it with barbed wire fencing, take the help of police if they object or agitate.