Dearest is not suitable for wedding song.. (sad meaning & sad melody)
Just like Ayumixx mentioned above, most of ayu's songs are one-sided love, about life, etc..
My suggestion, based on lyric: Voyage (planned to use this song too when i got married
)
As for the happy "neutral" song, you can try:
HONEY
Blue Bird
Greatful Days
Glitter
Sunrise ~LOVE IS ALL~ (or sunset ~LOVE IS ALL~ if you want to sing ballad)