1. Is this valid (including the two breaks of journey)?
Yes; you are remaining within the date-wise validity of each portion; none of the tickets you have proposed have any break of journey restrictions, and it is therefore permissible to break your journey an unlimited number of times during its date-wise validity.
2. Is this the most cost-efficient way of doing things?
Certainly not. For a through ticket, possibly, but there are definitely cheaper combinations of tickets (see below).
3. Is the correct price for the excess for the return journey £7.35 (i.e. the difference in price between the two tickets, divided by two)?
If you are obtaining a change of route excess from the Anytime Return route not via London, to the Off-Peak Return route via London, on one leg only, yes that is the correct calculation. Note that it is only change of route excesses that are calculated in this way. Other excesses, including overdistance, standard to first class, and change of ticket type (e.g. Advance to Off-Peak, Off-Peak to Anytime) are not calculated using half the difference.
4. Would my journey from OXF to London (i.e. the remaining validity of the outward ticket plus the first part of the return ticket) need to be to Paddington, stopping at DID (and not valid to Marylebone)?
For that ticket, yes you would need to travel via Didcot. You would, in fact, have to take a train that stops at Didcot (or you would need to change there), as you would be changing ticket portions there.
5. Would the excessed ticket be valid on the Underground (and have a Maltese cross)?
Yes, it would; note that because of the matter of Underground validity, it is not possible to obtain a change of route excess which involves going to or from a route which is valid on the Underground, from a number of train companies' guards onboard the train (even though there is no penalty for obtaining it onboard the train). This is because these guards, such as on companies like CrossCountry, Northern, Chiltern and Virgin, have ticket machines which use 'loo roll' tickets which must be scanned at the ticket barriers using the barcode reader, or alternatively inspected manually. London Underground do not accept such tickets on their services, due to the fact that it could cause lengthy queues and delays at the barriers at busy times.
You can also not obtain such an excess online. It is therefore safest and easiest to simply obtain a change of route excess from the ticket office before travel; this way you can be sure that you will actually be able to obtain the excess.
As for a better combination of tickets as alluded to above, here would be my suggestion.
Use the CrossCountry site to buy a combination of Advance tickets from Derby to Coventry and Coventry to Banbury, and then an Off-Peak Day Single from Banbury to Oxford. You can book these tickets for the 07:06 from Derby, which is a direct service to Oxford (it departs Coventry at 08:25, departs at 08:55 and arrives at Oxford at 09:15). This should cost a total of £33.50.
From Oxford to London, you have a variety of options. The cheapest would be to obtain an Advance ticket; the cheapest Advance tickets are available on Chiltern's quick, half-hourly service to Marylebone. These are widely available at their lowest possible price of £5.40(!). It is also possible to obtain GWR Advances, although these generally involve changing at Didcot or Reading, and are generally more expensive (although, despite the change, the journey time is almost exactly the same).
If you book on the CrossCountry site, it is possible to change your Advance without the usual £10 administration fee (although, in the case of an Advance, this must be done prior to the scheduled departure time of the service you are booked to take). This means that, if on the day you realise you won't make your booked train (or want to take an earlier train) you can still change your ticket to another train, which would likely involve changing it to an Off-Peak Day Single, which costs £26.50.
Note that, if you have obtained paper tickets, you will need to return these by post before you receive your money back for the old Advance. If you obtain e-tickets, these are simply deactivated remotely and you can be issued with new e-tickets or paper tickets without having to send anything back. It is possible to print out e-tickets to avoid the problems that could result from your phone etc. running out of battery!
From London back to Derby, against Advances are the most cost effective option, with prices hovering around the £20-30 mark, with First Class only about £5-10 more. But naturally, this ties you to your booked service. If that is not acceptable for you, there is of course the option of purchasing the Advance through CrossCountry as above and then either changing the Advance if your plans change, or simply changing to a flexible fare. But if you know you will require flexibility and don't want to tie yourself down, the Super Off-Peak Single fare from St Pancras to Derby is £65.50; this is valid for departing St Pancras from 10:30 onwards. This is very poor value in comparison to the Super Off-Peak Return, which costs just £1 more, and thus if there is any likelihood you will be travelling back down to London within the month following your journey home to Derby, it would be advisable to obtain the Return.
There are certainly cheaper tickets available which are valid out of St Pancras after 10:30 (such as a Super Off-Peak Single from King's Cross to Doncaster, bought on the likes of LNER's website, which would cost £46.60). However, depending on the machine at which you pick up your tickets, this may incorrectly print something referring to an "LNER Discount". This may result in difficulties at the St Pancras barrier line, or onboard the train - even though the ticket is valid on any train company, and is valid via Derby. Whether or not you're willing to undergo this potential hassle for the amount saved is up to you.
And if you need to travel before 10:30, a simple Anytime Single to Derby is going to be your cheapest option (there is no Off-Peak Single, and even if there were, there are no trains to Derby which it would be valid on, which the Super Off-Peak Single isn't also valid on). If you end up holding a Super Off-Peak ticket and needing to travel before 10:30, you can obtain your excess for the difference (£32.00 if you have a Super Off-Peak Single to Derby; £59.40 if you have a Super Off-Peak Single to Doncaster) - there is no need to buy a brand new ticket. You can, in theory, obtain your excess at any point, including onboard the train. However, the barrier line at St Pancras are unlikely to let you through without an excess, and so you'd need to obtain your excess before travel, or at the St Pancras ticket office.
I know the above is a rather lengthy post, however hopefully it's covered all aspects of your journey and the tickets for it.
