On the T35 (Mk1), the scraper rings were of the slotted type, whereas later Mks employed stepped scrapers. The standard reccommendation for ring gaps on air cooled engines is .003" per inch of bore - which equates to .007" on the postwar Douglas. Measurement of this gap may not be easy on the Mk1's as some of them had scarf joint rings.
The fit of the inlet valve in the guide will obviously affect oil consumption, but should not result in enough excess to oil up plugs, etc. After all, the Douglas engine relies on mist lubrication for the valve gear - there is no positive feed to the cylinder heads.
As has been stated before, the right hand cylinder always suffers first - because it has better lubrication due to the way the oil is thrown from the rotating crankshaft - up into the right hand cylinder and down into the left hand, resulting in the top of the left hand cylinder being somewhat starved of oil (prewar engines had a small hole drilled in the front barrel to provide an auxiliary feed to the piston skirt).
Another avenue that might be worth investigating - is it the oil rings that are at fault or are the compression rings allowing too much blow by - thus creating crankcase pressue and forcing oil past the rings (and guides).
To relate my personal experience - since rebuilding my Dragonfly (about 13 years ago) the oil consumption has gradually got worse, but has never resulted in fouled plugs, and the bike has proved to be utterly reliable. Eventually, of course, it started to leave the dreaded 'blue trail'. On inspection, the pistons looked OK, so new rings were fitted, along with new valves and guides - no improvement whatsoever! Then a well respected club member said "Stick an old set of pistons in it - you may be pleasantly surprised". With this in mind, I raided my junk box and sorted out a pair of well used, but serviceable pistons, fitted them and, hey presto!, no smoke and the oil consumption is reduced to about a quarter. Maybe the moral of this story is " Don't rebuild for rebuilding's sake - leave well enough alone!"
Regards,
Eddie.