Cordelia and Leon IV question

Cordelia is not ideal without a +atk or +spd nature. Not familiar with Leon, and maybe others could help you on him.

They aren't very good with those IVs but Leon is still usuable with a BraveBow+ , he will be similar to Gordin. However Cordelia suffers from it a lot, and won't be very effective in high arena tiers.
But you can still build her as if she had a better IV set and wait to get another one to merge her into the new/better one. This way, she can help you to clear the compain/missions/PvE , in waiting for using her in Arena as well.

===Insight for you to comprehend things better===

As a general matter, SPD improve HP's value.
So having +HP/-SPD ivs is often a very bad combination (except for very slow unit that won't suffer from a lower SPD value, but still an Atk or Def/Res boon would have been better for them then).