1. Jess is probably referring to the 20% defense reduction that Blood Pact used to have. It was removed earlier this year, either with Rise or Neamhain idr.
2. Glide spam is a semi-viable method of damaging but when it comes down to it moonsplitter will always give you a higher output. You don't sacrifice moonsplitter, you're just not able to hold moonsplitter for more than 5-10 seconds. At the highest level, the core gameplay is still centered around moonsplitting with glides being used to reposition yourself. (party play, you get more aggro in solos or duos which demands more glides)
3. There will always be class disparities depending on the type of raid. The sad reality is that it doesn't matter how good you get, the best Lann will always lose to the best Fiona in Neamhain. This shouldn't discourage you from playing Lann though, if you master him you can still put up pretty respectable numbers in Neamhain, it's just a matter of practice.
I would suggest finding a friend or two that would be willing to load up a practice run with you. Get past the first stage of Neit and run 10-8 until you feel comfortable. She only gets one new move in future stages which is her one hit KO, so if you can get 10-8 down you should be set. You're most likely are still playing to "survive" and not playing for efficiency. Just keep working at it, there's probably only a handful of Spear Lanns who have a proper grasp of how to approach the raid.
You'll find that one of the biggest problems with Lann in party play is his lack of mobility. If you run with people who hug the boss his damage goes up by a quite a bit.