For route planning and accommodation along long distance trails, i often use http://www.nationaltrail.co.uk/.

The swcp has its own dedicated site as well, and their publications are very good for accommodation, though i admit I've never planned for groups of 6 (i normally walk solo).
I recommend that you get the bus from penzance to mousehole and start your walk there as the walk from penzance to mousehole (pronounced mowzl) is all on pavement. If your time permits, an extra day in the St. Just / Bottalack area would be lovely for seeing old tin mines and for te art galleries. St. Ives is great too, but its more of resort town, whereas the others are villages. Accommodation between Penzance and LandsEnd in either in Porthcurno or Treen. Here are a few places in my book (haven't used any of them, since i was camping/backpacking):
www.relubbushouse.co.uk
www.cornwall-online.co.uk/trendrennen
www.porthcurnohotel.co.uk
www.seaviewhouseporthcurno.com


http://www.ramblers.org.uk/ is also a good resource for walkers accommodation and luggage transfer companies.

>>Should we include a stopover at Exeter?
not really. If there's a big gap between train changes then, yes, pop over to see the cathedral. Otherwise, the city itself is quite missable, atleast I wasn't impressed.

If you do decide to spend the day in the Salisbury area, there's a lovely walk from Salisbury to Stonehenge via Old Sarum. No matter how crowded Salisbury, and Stonehenge are, the walk is always quiet and goes through some lovely rural country. The walk is about 10 miles and you can get the bus back from Stonehenge to Salisbury. The bus also stops at Old Sarum and you could skip the walk from Stone henge to Old Sarum, because, while it's nice, it also goes through lots of stinging nettles (really, really hate them).

If you decide to do parts of the Ridgeway, the section from Avebury to Marlborough and the bits through the chiltern hills are my favourites.