Hi kijeta,
Yes, there are some anomalies at the edges of the Oyster area. The train companies accept that anyone using Oyster in the correct way will be charged according to the fare finder, so it’s not really a grey area. If a travelcard covers the zones shown on my fare finder for the journey made then no extra will be deducted.
In your specific examples, Ewell East to West is described as being a zone 3-6 journey, so a travelcard (on Oyster) would be valid to travel via Epsom with no extra charge. A paper travelcard would not be valid, but I prefer to view that as a benefit of using Oyster. Slade Green to Crayford is well known to me and the same applies, except that there are direct trains between the two which stay within zone 6. If you do travel via Dartford then the same fare would be charged as that is the zonal coverage for that journey.
Did you mean Turkey Street to Enfield Lock via Cheshunt? In that scenario the single fare is described as zones 6-8, so you would be charged extra if you had a zone 1-6 travelcard. If you want to stay within the six zones then you need to travel via Tottenham Hale and Seven Sisters using the Victoria line. It is also possible to buy a paper zones 1-9 travelcard by specifying the origin station as Amersham or Brentwood.
In terms of the Freedom Pass or 60+ Oyster card you do need to stay within their specific areas. Whilst you would get away with touching in at Ewell East and out at Ewell West, if an inspector checked your card en-route then you would be treated as having no ticket. Dartford has historically always been in the Freedom Pass area, so that isn’t a problem. Cheshunt is valid on a Freedom Pass as long as you use the Overground services.