Exeter-Okehampton services have no passing time at Yeoford, so as far as journey planners are concerned those trains do not run through it and won't offer split tickets at that station.
I don't think there are any sites that will currently offer non-stop splits. And in any event the existence (or otherwise) of a timing point in a schedule isn't determinitive as to whether you can split there. Coming back to our earlier example of Tamworth, it's not a timing point for trains that run through non-stop - yet nobody would suggest this means you can't do a non-stop split there.
At the end of the day, I would have said the existence of platforms or timing points, or indeed the signalling arrangements, are all red herrings. They are operational considerations that the average passenger cannot be expected to know about, and indeed which aren't referenced in the NRCoT or any other internal or public document.
What an average passenger can rely on is their own eyes, or the likes of Google Maps (other mapping services are available). Both would confirm that an Exeter to Okehampton train passes through Yeoford. Therefore I'd say a non-stop split is valid there.
I'd say the fact that no Okehampton to Crediton trains can physically call at yeoford would mean that a split ticket would be invalid
The only reason it can't call there is because there isn't a platform on the line used by Okehampton trains. I don't see how that changes anything; as we've established, using a non-platform line with the likes of Tamworth LL is absolutely fine on a non-stop split.
any Yeoford to Okehampton journey would be routed via Crediton
If only Okehampton trains called at Crediton, meaning you had to change at Newton St Cyres or Exeter to get from Okehampton to Barnstaple, would that change anything? I don't see that it would. The timetabled calling patterns aren't relevant to non-stop splits.