After getting the cold shoulder from more than a handful of vocal-music "pros," I've decided to be proactive with my opera and take a hard look at it myself.
Being an instrumentalist, I take for granted that we can hit any note out of the clear blue at any point in a piece of music and play an independent melody, but singers are a different breed. As I understand it, they need cues and hints throughout the accompaniment to stay on pitch and in place so, with this in mind, I've started revising my score. I've been adding more cues, adjusting both the accompaniment and vocal lines, and, in some especially tricky spots, letting instruments double the voices.
I hear Beethoven had the same issues.
I've worked my way through most of the first act - I plan on finishing the last scene of Act I tonight - and from there I'll make a new piano score. The hard part is going to be the choruses. I'm not sure how to handle those yet. The good news is that since starting this round of revisions, I've heard back from two of the "pros" that I've contacted. One said he could meet in about two weeks and another said she might be free in mid-July. I hope it will work out and that I'll have some newly-revised music to show them.