MW switchoff isn't going to happen without notice - over the summer is more likly due to sports rights anyways.
As rights come up for new agreement's, online will inevitably be included, some rights may drop to other broadcasters, and dab opt outs could be introduced.
With north Devon DAB being on a separate frequency you could have....
Plymouth and FM - No change
N Devon DAB - No Change
Exeter / Torbay DAb - Exeter City and Torquay United on separate streams (see also Five Live)
Online - Normal Programming.
South Devon is then treated the same as Plymouth, I.e. no Normal Programming, or the BBC could leverage additional capacity on the multiplex to carry BBC Devons regular programmes.
DAB+ opt outs are also a possibility, I.e. BBC Devon on original DAB 64kbps/mono, BBC Devon Exeter City and BBC Devon Torquay United in DAB+ 32kbps. For sports commentaries you might be able to get away with an even lower DAB+ bit rate and shave more off the normal programming to squeeze Exeter Chiefs in too.
If it's anything like BBC Lancashire used to be, not all the commentaries clash every week anyway (and do the the Exeter teams really play at the same time? The three Nottingham ones never do)