Hi dontknow. I agree with all the suggestions above.
Your dog sounds new to you, and his problems with being left are probably very deep-seated, so he is going to take lots of work.
Our Border Collie took 2 and a half years of serious sustained work to calm his separation anxiety! His behaviour was much like your dogs': toileting inside, chewing, barking.
Give him time, don't get angry when you come home and he's messed, and try EVERYTHING possible.
Ideally, as mentioned, you would start again from scratch-leaving him for literally a minute and building up over a period of weeks. If you work/study, can you take any time off?
I know you would be devastated to take him back, so I really hope you can work together and get it sorted.
Lisa xx