Also, what is the with using a combination of a Rover and a normal ticket. Does the train have to stop the the station where you change from one ticket to another?
Nope, the train doesn't need to stop with a normal ticket plus a Rover as the Rover is considered a "leisure travel pass" as per NRCOC 19 quoted below.
However, with the one day Ranger tickets (ie the Devon ranger) it's less clear. The Devon Ranger could
be considered a Season ticket as it allows multiple journeys to be taken on the same route using the same ticket.
With the Ride Cornwall ticket things are more clear cut as it's a ticket scheme run by Cornwall Council and there is specificity excluded from the definition of a season ticket.
Thus, you could go for a off-peak return between Bath and Tiverton Parkway and then a 3-in-7 Freedom of Devon and Cornwall from Tiverton to St. Austall and travel on the 08:57 direct service from Bath to St. Austall which doesn't call at Tiverton. The cost of this would be around £115ish (no Family Railcard discounts on Rovers so 2 adult tickets @ £40 each plus £35ish for an off-peak return BTH-TVP 2A 1C) but you would have the flexibility on the Sunday, plus unlimited travel in Devon and Cornwall across all three days.
19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover the entire
journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit their use);
(b) the train you are in calls at a station where you change from one
ticket to another; or
(c) one of the tickets is a Season Ticket (which for this purpose does not include
Season Tickets or travel passes issued on behalf of a passenger transport
executive or local authority) or a leisure travel pass, and the other ticket(s) is/are not.
You must comply with any restriction shown on the tickets relating to travel in the trains of
a particular Train Company or Train Companies (see Condition 10).
If you do not comply with this Condition, you will be treated as having joined the train
without a ticket and the relevant parts of Condition 2 or 4 will apply, either to the entire
journey, or from the last station where the train stopped at which at least one of the
tickets was valid.
For the purposes of this Condition, a “leisure travel pass” means any multi-journey ticket
(excluding Season Tickets) valid for:
(i) at least 7 consecutive days; or
(ii) at least 3 days in a period of at least 7 consecutive days
and includes rover tickets, travel passes, flexipass tickets and Britrail passes.
Edit: I see just posted while I was writing my reply! I'll leave it on here for anyone else who's thinking of doing something similar in future.