Actually you touched on a few issues.
Certainly "vegetarian substitution" is ok to do.
But what you said about "incomplete without the lamb course" is very terribly wrong. Whoever told you back in the "campus group more than 20 years ago" gave you very terribly wrong information.
Meat is permissible to be served at the Passover Seder (ritualistic meal) but that it is strictly prohibited to serve roasted meat. By what you suggest of "lamb" it appears you're attempting to replicate the "Passover Sacrifice". Because the "Passover Sacrifice" is strictly prohibited except in the Holy Temple, it is strictly prohibited to eat it or to serve any meal item that resembles it.
The Holy Temple hasn't stood for approximately 1,950 years. No "Passover Sacrifice" has been made since then.
There is however a ritualistic "roasted shank" item utilized for the Passover Seder but that item is not eaten.
Also very recently another Y/A R&S member asked:
"How can a christian celebrate passover & hanukkah?"
by "bhk91210"
https://answersrip.com/question/index?qid=20080411085347AAECLX1&show=7#profile-info-EVEJFBQ6aa