Your case is far from straightforward. There are a lot of factors at play here, I think. Firstly, you state that the poor dog has "moved several times". That in itself is stressful, plus the fact that dachshunds are quite nervous dogs and easily stressed (I had one as a child). Secondly, you have written that your girlfriend's dog "now lives with some younger males in their 20s who dont seem to have much time for the little guy". Is she sharing a flat or house with these males? if so, do they have access to the dog whilst your girlfriend is out? Could one of these young males perhaps not like dogs and is locking him up or forcing him into the garden to avoid contact when she is not there? Have you looked into that possibility? It is not likely, but I mention it in any case. The sudden changes in behaviour suggest some "behind the scenes" factor.
Being neutered can affect a dog's behaviour, but as the traits you mentioned existed before the operation, it is unlikely to be the cause of the problems. However, partial incontinence can be a rare side-effect of neutering. It might be worth consulting the vet about that issue.
You are clearly a kind and compassionate person, having gone to the trouble of asking for advice on your girlfriend's behalf. Is there some reason why she is not seeking advice herself?
It is difficult to offer any obvious explanation. It would probably be wise for your girlfiend to call in a dog behaviourist, who will go into these and similar questions in depth, maybe watch the dog in his environment. That is the best way forward.
Every best wish to you!
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