Since you decline to share the Chinese prior art with us, some of what I write is speculation. Many, if not most, people posting questions here assume patents cover much more than they actually do. They see long complicated claims and assume they are broad when in actuality they are narrow. The first and most basic question is whether an expired Chinese patent (or even a published application) is considered prior art. The answer is absolutely yes. It doesn't mean that the patent examiner would have found this prior art, but since you found it, you are legally required (at least in the US) to tell the patent office of its existence.
What i wonder is: how to get an authoritative opinion that could be
worth something, like if my patent, should it be accepted, could be
enforced in court provided this related prior art?
The only way I know of to get an authoritative opinion is to hire a patent attorney. You are asking for a legal opinion and only a licensed attorney can provide that.
I read of plenty of granted patents that aren't enforceable, but i do
not know if the reason is only very narrow or limiting claims, or
simply claims that won't be deemed as valid in court.
I'm only speculating, but my guess is the main limitation on enforcing a patent is narrow claims. Many people who write their own patents rather than hire an attorney do a poor job of claim construction or else don't do an adequate job of defending those claims with the examiner. There are some bad patents that get granted, but my guess is those are fewer than many think. In any case you state the Chinese patent you found is expired so it isn't a problem for infringement. It is still prior art.
Theoretically, if i restrict the claim A from the previous patent to
be A + B, should my application and claim be fine? (I happen to know
that just A is not useful). In essence i would be teaching a different
use for a slight modification of the prior art.
Maybe or maybe not. It depends on whether A + B is legally obvious. Also, there might be another patent which covers A + B. The best you can hope for will only be protection for A + B. This is where working with an actual patent attorney is most important. If you are asking this question, then you really don't know how to construct claims. Since you don't understand claim construction your specification probably isn't written so support those claims. What all this means is you are probably saving money but not getting what you want which is an effective patent.
I am not a lawyer and this is not legal advice. I do, however, have 90 patents to my name so I've been through this process before.