Welcome kellbell. My DS has diplegia and is slightly younger than your DS. We got the diagnosis in February, and at the time I wasn't sure I believed the Paed, but slowly I got used to the idea. And it is honestly fine. i soemtimes struggle with other children of his age about the place, but none of them are as lovely as my DS and some of them are just plain boring. So what if DS can't walk yet. One day he will, not sure when but he will. He is also DS just like he was before he was diagnosed and IMO the loveliest smiliest child I have ever met. He can't really talk yet either.
You also have had splints so that presumably gets your DS into the system, and again i presume means that you have a physio. IME the physios are the key difference between good treatment and rubbish treatment, and so much more use than the Paeds, who even in my large teaching hospital rely on the physios for diagnostics etc for little children. So if you can short circuit the rubbish paeds and go to community healthcare for physio (I assume seeig as they are suggesting SN groups and a nursery that you can push that through those avenues if the paeds persist in ebing useless)
My Ds also mainly drags himselfs by his arms (much more so since he got the splints) but can crawl normally, its just a bit too much effort for him most of the time as he is very quick and the drag method.
On the questions of what age DS did things, I would actually probably have to check the previous posts I had on sitting (which DS also doesn't like, he much prefers to kneel) but he started the commando crawl dragging just as he was last discharged from hospital (13 months corrected age) and cruise at 15 ish months corrected. He is just trying to stand independently (and fall over) at 18 ish months now. But its hard work. He gets set back every time he is ill which is a lot as he's in nursery and also has chronic lung disease (from being prem). But personally i think he does that when he's trying to tell me he's had enough now. As for where DS sits on mild or moderate, in our case they won't tell me anything until he hits 2 corrected age next May. I have an idea but I have to hope for the best and expect the worst if that makes sense.
Have you applied for DLA yet? If not I would recommend you do as it took us 5 and a half months before the DWP agreed, and although I felt that it was a bit of a shame if DS is entitled to it he's going to have it.
anyway there are peopel far more knowledgeable than me on this thread, but DS and I are similar(ish) newbies and finding our way through the system.