Giving her staves is still more of a comedown when Staff only units such as Aideen could profit much more by Silence/Warp/Return's rather high exp count.
That is true but they have more time to get the EXP. There is no long term penalty in training Diadora. I managed to get through the game with enough turns to hit the A rank in speed getting Diadora to the high 20s and Lachesis was promoted in time for mid-way though chapter 3. A number of turns with Warp, Return, a dancer and possibly a spare unit is all you need...
Also, at the beginning of each chapter, I have the units with 40000 or more buy the Elite Ring, send them through the arena, sell off the Elite Ring, and pass it off to the next unit who could afford it, ending with the last unit being the unit I intend to hold the ring for the rest of the chapter.
Usually the best way to do this is do it when more people can afford the Elite ring so the last person doesn't have to buy it twice (or miss out on a bout)...
Personally I like having Ethlin as the last person which kind of screws over generation 1 in EXP but gives Leaf an easier time (though it is a bit overkill). Some runners favor Sigurd though so Celice can be promoted after clearing the arena in Chapter 7
and thus dismount becomes useful for once since unmounted Celice has better movement in the desertAs for who gets the Elite Ring first. Personally Finn is usually the one who gets the kill and doesn't need it anymore come start of chapter 3
after doing the arena bit