
Several problems here:
To sort out the seperation anxiety would need the compliance of the neighbours because you will need to go back to scratch and build up his confidence that you
will come back. Unfortunately a dog with a needy temperament and an owner who seldom leaves him, there will be great trauma when a problem like your hospitalisation arises. Your statement "I NEVER EVER leave my pup for long" tells a story...great when you can be home all day but we will occasionally have these unplanned problems.
Neighbour problems of the 'drip drip' type can become one drip too far. Beagles are not the quietest of dogs and a couple of hours of him letting rip with anxiety could have been too much to bear
I feel that you need to talk to them, try to get them on your side and tell them that you intend to get help with his Sep anx and that it
will get better.
If they realise that it may be quicker to sort the dog than get legal action, they may stop complaining.
I dont know your rights because you could be a home owner or tenant private/local authority?
I know it seems the last thing you want to do but please try to negotiate with your neighbours......they may be rightly fed up......I probably would be and I love dogs...just not barking dogs.
A good behaviourist will work with you to lessen his sep anx symptoms. Vet might also suggest things to calm him whilst his behaviour is being reshaped.
If it's any help I have the worst sep anx candidate here in my house and I can now leave him for long periods and he totally relaxes..snores in fact...it can be done.