'Via Norwich' is a somewhat odd restriction, as it appears that tickets from Ipswich to Great Yarmouth can still be sold with an itinerary via Lowestoft, so long as the subsequent change is at Norwich, and not Reedham or Brundall.
Many of the trains on the East Suffolk Line run as through trains from Ipswich to Norwich, even though they aren't advertised as doing so. Indeed, at some times of day, the whole journey from Ipswich to Great Yarmouth via Lowestoft and Norwich could be on one train. An example being the weekday 1617 from Great Yarmouth, where the unit then works to Lowestoft, then from there to Ipswich. However the journey time is very much slower than going via Diss.