If you don't use the adaptor you fit no.19, instead of no.26.
If you don't use the adaptor you only fit no.19, instead of nos.19 & 26.*
But I don't see why threadlock and regular screws couldn't be used, at least in the case of the forward screw, which doesn't have a safety clip fitted in either case.
Especially at £6 a pop.
However, it's not my bike so I'm inclined to err on the side of caution...
*Edited for clarity.