Given the fairly short distance between Lea Valley Campsite and Farm Hill Road. Just 2.3 miles! It shouldn't be about who's patch or who's responsible for it? It's a distance any bus could cover quite easily, it could probably cover that section in 10 minutes as it would rarely stop inbetween. But then again when I've used the 215, I've noticed people who seem to live in Sewardstone walk to the campsite to catch the 215 so demand must exist. Other than the village, there are pubs, care homes and the Gunpowder Park plus Waltham Abbey, which has 23000 people. The demand for buses there would be decent had it not been left to the whims of the private operators.
https://www.change.org/p/bring-tfl-buses-back-to-waltham-abbey-fix-our-inadequate-public-transport
A petition attracted over 1500 signatures, enough for the London Assembly to vote on it and they supported it. Now it's on both TFL and Essex to come to an agreement, why this isn't straight forward is beyond me. Does Essex help fund any TFL routes at all, Loughton and Debden get the 20, 167, 397 and the W14 (formerly 549) was recently enhanced to an hourly service, extended to Whipps Cross and now also runs on Sundays. Only the Loughton/ Debden town service and 66 are non-TFL there. This is in the same district as Waltham Abbey and no reason, it shouldn't receive the same treatment. Your idea on the 385 really shows any number of possibilities could easily fix this issue which show how useless our politicians really are.
It's not like it has to cross a bridge and the bridge collapsed, it is a normal road which can get quite busy that just happens to cross some arbitrary line we made up. Also quite right to mention the other cross-border towns, it really should be consistent, so every town up to 5 miles or so from the London boundary received a TFL service, would hopefully fix the anomaly of Potters Bar which has a large bus garage but few of the routes actually serve the town. Go to Cuffley or the road to Barnet, almost always an out of service TFL bus going through the area at seemingly any time of day, how can that be right?