Casting delay (interruption) after moving/stopping
 

Simply, there wasn't any on the Live servers at any point. What "delay" there was was simply a side-effect of however the client/server handled movement (either lazy about telling the server you stopped or the server would keep predicting you were moving). Two common techniques to "force" the server to treat you as stopped was to stop and either turn back and forth real quick or use an insta-clicky. 99 times out of 100 you could cast instantly and never make a channeling check (95 out of 100 when levitating, and for the remaining 5 the movement was registered as very minor so the channeling check almost always succeeded).

Right now it seems you need to be stopped for something like a good two seconds before casting on here or you are almost guaranteed to be interrupted from "movement." Considering how quickly the server responds to/processes commands normally I'm assuming this delay is programmed in, but if it's a technical issue and can't be fixed it would be nice to know that too.

I don't have that problem. Its the same as I remember it in live, if you stop and do a tiny turn, spells cast fine.
look at your latency (f11) and see how laggy you are

I have done what worked on Live via the stop and turn slightly. This has been sufficient for me to successfully cast on P99.

I don't know how accurate the F11 graph is but I've been sub-75ms on it every single time I've checked (outside of when the server was getting DDoSed of course). But the latency shouldn't matter anyway as long as the "turn" arrives at the server before the "begins casting" (at least, it didn't on Live), and barring some packet loss or something that shouldn't happen.

I asked casters in my guild (that I know played casters on Live) and I had at least 10 people say they were seeing the same issue I was. So either our guild has the unluckiest casters alive or something is off...

I have noticed it do a degree, character has totally stopped moving(graphically) from running and a spell is casted but still seems to get interrupted, but its not really game breaking or anything.

This... same as I had to do on Live and works perfectly on P99, even with my 200+ ping :O