Here's my take on it........
Cont ignition is good enough for almost all situations. Boeing kindly thought that with the early problems with engines we had better have back up igniters. So they then put a switch so we could select either set to test them individually (on start up). However, if there was a flame out, it would be nice to have every thing available operating to get the engine relit. So as well as the CONT selection they put FLT in there to save us having to move a switch to the both position.
An emergency descent is no defferent than a normal VNAV PATH descent other than the higher ROD and MACH number. So CONT should do the job. I think the "Start Switches.......ON" means that as long as some form of ignition (cont or flt) is selected, the egines will be sufficiently protected.
A somewhat garbled reply, hope it makes sense.....
Mr L