Hi Neil,
Before I get into specifics let me say that if you have been getting refunds for walking between nearby stations which are not set up as interchanges then you are very lucky. There is no such concept of reasonable interchange, either a pair of stations is linked or they’re not. You can of course request that they consider setting up new pairs, but I should caution you about their unwritten rules. Firstly almost all interchanges are less than 800m. The ones that are further usually involve National Rail stations or very specific local circumstances. And Secondly there must be significant journey opportunities enabled by the interchange that aren’t catered by other connections. That said, let’s look at the pairs you have suggested.
South Hampstead to Finchley Road or Swiss Cottage: Finchley Road is too far, but I agree that South Hampstead to Swiss Cottage ought to be an interchange, especially as it’s shown as a dotted line on the tube map and is only 350m. Having said that, its use would be quite limited to short local journeys, so I’m not sure whether it would get added.
Warwick Avenue to Royal Oak: Frankly no chance because the next stop on both lines is Paddington. The walking distance is also 900m.
St Johns Wood to Maida Vale: 1.2km apart, so forget it.
Rusilip to West Ruislip: There is a connection between West Ruislip and Ickenham so you aren’t going to get another. They only allowed that one because of the lack of local buses in the area.
Preston Road to South Kenton: Again, the shorter distance between Kenton and Northwick Park is an interchange so you won’t get another longer one.
Edgware to Canons Park: At 1.6km it would be far and away the longest interchange. Need I say more?
Now, moving onto what you say about yellow readers. The rules say that you must touch in at the start of a journey and touch out at the end. Sometimes there may be opportunities to join journeys at nearby stations, but for that to work properly you must touch out before the walk and touch back in at the end, having completed the walk in under the alloted allowance (usually 20 minutes). If you attempt to walk between two random stations without touching out and in then you will not be charged correctly, and could potentially be accused of fare evasion.
I don’t quite understand what you are saying about Finchley Road and West Hampstead. There are interchanges between all three stations at West Hampstead and also between Finchley Road and Finchley Road and Frognall.
If you give me some specific start and end stations for journeys you make then I can try and assist with options. Sometimes it has to be accepted that a bus is probably the best way.