Need help with Morgan (F)

I myself have run both builds (blarserpent defense and blarblade offense) and both are good. She wont be doubling alot of people (36 speed is meh unless you find ways to boost it, IE fury, speed +3 S seal, could potentially hit 42 speed with hone fliers/spd tactic (which is far too rare)) so long as she's buffed she works out nicely. Her blarserpent build (and one I've yet to complete) is very fun and if done right makes her a tank (although my IV's are off for a defensive build, so I'll leave it at that.

In short, yes, it can work, and it works nicely if you give her the buff support she needs. I say go for it. You don't lose anything by giving her blarblade.

Do you prefer to play her offensively? I personally find her spd quite underwhelming for a blade mage (mind you, flier buffs, fury, and a spd+3 seal can make this almost irrelevant) and like her potential as an anti-mage. However, if you want to play her with the blade tome, I think it's also a correct and good decision. Spd ploy is reasonable; goad fliers will probably do more for the team (goad and not fortify, because she needs to receive the fortify buff, which will also give her a good chance to survive a counterattack from anything except an axe, arrow, or green dragon). Desperation is the usual B-skill to go with fury on a blade mage, and I don't see it being any different here. Just make sure she's receiving hone fliers and fortify fliers, and maybe an additional goad if necessary.

And on a flier team I do think reposition is better. I asked about ardent sacrifice a few days ago; people speaking from experience stated rather consistently that it's a very slow method because the battle is well under way by the time you'll get to attack - somebody has to tank a hit, and then you still have to give up your turn. Probably not how you want to do it as a flier.