Best dragon to inherit fortify dragons to?

If you’re using the other Ninian, I’d say merge them. But that’s just me.

A single point of Spd is never a reason to merge. Merging is for arena/AA scoring.

In that context, you need to find out whether you need the extra score on Ninian. Your team seems very temporary. You need to think ahead what team (and associated skillsets) you are going to field in the long run. (ie. +10 merge) Assuming final build and abundant resources, as well as the same heroes on your team, Fortify Dragon won't go to Fae, who can run a ploy skill. (She also won't be -Def.) Tiki can be good. Nowi might need to run Panic Ploy. If it is a full dragon team, certainly another Fortify Dragon is helpful. But scoring potential is still the hard cap here. If you can't get enough scores, it doesn't matter whether an extra Fortify Dragon helps you win more.

Thus, your best option seems to be saving the extra Ninian for when your main team is more set and more merged.