You could have bought a Boundary Zone 6 - Milton Keynes Off-Peak Return (out/rtn valid after 0930, no afternoon restrictions) for £31.20 which I guess would save a few pennies over two Anytime Singles at £16.80 each. But there's not much in it!
The 0900 or 0935 London Northwestern services, due into Euston at 1007 and 1017 respectively.
The shaded area shown in the map linked from this page.
Not quite. You can't use Avanti services as a 60+ Oyster isn't valid on them, even in conjunction with a ticket to/from Boundary Zone 6. A Freedom Pass is valid on Avanti, however.
The additional restriction only applies to journeys wholly within the Network Railcard area. As Cardiff is outside, you can use a Senior Railcard at any time.
As a general rule of thumb, for journeys heading towards London the restriction is based on the arrival time. For journeys heading away from, or across, London it's based on the departure time.
Since you want to take the Milton Keynes stoppers which all call at Bletchley, and since you are returning the next day, you would be better off splitting at Bletchley. This is because Bletchley is within the 'single leg pricing' Project Oval PAYG area, so a single costs half of the equivalent Day Return, whereas the period/open return you could get to Milton Keynes is more expensive than a Day Return.
The admin fee for refunding your tickets would depend on the retailer you used. Most retailers charge the maximum £5 admin fee allowed under the National Rail Conditions of Travel. Some charge less for lower-value tickets (though this usually only applies to tickets under £10 or so). A few don't charge admin fees at all - Railsmartr and Ticketyboo are the only two that universally apply this for walk-up tickets, to my knowledge - so it might be worthwhile using them in future.
The saving doesn't apply to tickets from Boundary Zone 6 - the OP would be better buying the Bletchley ticket to start from Euston to get the prices you have mentioned. Harrow is also more expensive than Euston!
I am a tad confused about your observations regarding freedom passes and Avanti as the freedom pass map shows it being invalid on National rail north of Harrow and Wealdstone which is Zone 5
Unless you change to the Overground at Watford Junction it looks to me that you would not have a valid ticket between Hatch End and Harrow and Wealdstone if you hold a BZ6 and a freedom pass/over 60 but I am open to correction if I have misunderstood
The map is a little misleading, in that it shows the stations you can travel to with the pass and, as London Northwestern do not stop at a station beyond Harrow & Wealdstone within the zones, it cannot show the zone 6 validity. As there is likely to be an admin charge on refunding the tickets you already have (you haven't mentioned where you bought them) there is unlikely to be any benefit in refunding them on this occasion.
Ah this makes sense but the question is "is the over 60 pass valid on LNW with a BZ6?"
Virgin Trains = Avanti.
You can use over 60 if you change to the Lioness line at Watford although this is much slower
