To swap a geared bike to SS roadside, the chainline has to be just about dead straight and the chain tension has to be in the 'Goldilocks zone' - not too loose, not too tight. Optimising for these factors is likely to result in a non-optimal choice of gearing, especially with vertical dropouts. So be it.
If the chainline is at an angle, it will keep changing gears while you are riding and either tightening or loosening the chain.
If the chain is too tight, you are much more likely to pop a chain side plate or a hub or bottom bracket bearing. If it is too loose, the chain will wander onto other cogs at inconvenient times or skip.
In July, a fellow PBP Audax rider destroyed his Campag rear mech and bent the Vitus Al dropout hanger so far that it clashed with the cassette. I bent the hanger back far enough to clear the cassette, removed the rear mech and shortened and rejoined his 10sp chain, all with my multitool. My spare 10sp quicklink had already been used and apparently he and the support crew couldn't purchase another. He rode that arrangement for a day and a half to the finish, though I made him climb in the saddle only, because I expected the chain to part at any moment.
