I don't know whether I have anything constructive to add to the debate, but I'll give it a try.
SPOCK: Decoding memory banks. I'll try to interpolate. The Valiant had encountered a magnetic space storm and was being swept in this direction. Swept past this point, about a half light year out of the galaxy, they were thrown clear, turned, and headed back into the galaxy here. I'm not getting it all. The tapes are pretty badly burned. Sounds like the ship had encountered some unknown force. Now, orders, counter orders, repeated urgent requests for information from the ship's computer records for anything concerning ESP in human beings.
I agree that "magnetic storm" could either be a colloquialism form "wormhole" or by the time of TOS they prefer the term "magnetic storm" over "wormhole" (regardless what the spin-offs say).
From the dialogue I take it that the "storm" and the energy barrier are two separate entities.
Where it really gets messy is the Enterprise's impulse speed and the colossal coincidence that Delta Vega is within their reach.
SPOCK: Recommendation one. There's a planet a few light days away from here. Delta Vega. It has a lithium cracking station. We may be able to adapt some of its power packs to our engines.
So how much time passed between their encounter of the energy barrier and their arrival at Delta Vega? If it's just a few days (Mitchell is getting stronger by the minute...), then the Enterprise
must have been able to almost reach light speed with its impulse engines. Can't say I like this idea (especially that with the space warp ability gone the ship probably won't intake enough space hydrogen for fuel
As for time dilation issues, maybe this line from "the Cage" is a hint:
SURVIVOR: Is Earth all right?
PIKE: The same old Earth, and you'll see it very soon.
TYLER: And you won't believe how fast you can get back. Well the time barrier's been broken. Our new ships can
(He's struck dumb by the sight of a lovely young woman)
...yes, go ahead. Can...? Can do what? Darn, just as it really got interesting he got distracted