Sorry to hear about your diagnosis.
Cramp - pretty much anywhere - is a very common symptom of MS. I’ve had it some pretty strange places, that I don’t think “normal” people ever get.
But basically, anywhere you have a muscle, it’s possible to get cramp. So I’d strongly suspect it’s all related, yes. If it gets bad, it’s treatable with muscle-relaxants, on prescription.
Twitches and buzzing, vibrating or electrical sensations are all common with MS too. Again, usually treatable, to some extent, if they become distressing.
The back pain is harder to say. “Bad back” is such a common condition in the general population, that it might or might not be related. Personally, I think I do get backache with MS. It has occasionally been severe (afraid to cough or sneeze kind of severe). But more often, it just takes the form of tiredness and only moderate pain, as if from standing for too long.
The doomy attacks could be anxiety. You say you’ve never had them before, but then you’ve never been diagnosed with life-changing illness before, have you, presumably? It’s still very early days. It wouldn’t be surprising if your body came up with some new manifestations of anxiety, in response to a situation you’ve never faced before. You could report it to the specialist as a symptom, and see what he thinks, but it will be interesting to see whether it calms down anyway, as you have more time to adjust to the news. I think it’s normal to have occasional feelings of panic, because this illness is so uncertain. But they may get less common and less overwhelming as you have a chance to come to terms with it.
Again, if you are panicking ALL the time, it’s treatable. But if it’s just now and again, you may not need to do anything more than accept it, and wait for it to blow over. It’s normal to feel scared of scary things, and an MS diagnosis IS a scary thing - especially at first. So don’t put pressure on yourself by telling yourself you ought to feel fine.