Are any immersion courses available for someone that never had a bat mitzvah...and at age 50 is this feasible?
Welcome and congratulations on your search and efforts!
First and foremost you should approach the rabbi in a local synagogue or Chabad house.
Beyond that, there is a wonderful organization called NJOP which developed classes which are delivered in hundreds of synagogues across the US and Canada, specifically they have a Crash Course in Basic Judaism, Hebrew Reading Crash Course and Crash Course in Jewish History.
There is no prescribed format for a bat mitsva in Orthodox Judaism, but it might include giving a short dvar Torah at a community Shabbat to celebrate a program of learning you have devised. More important, it should be the starting point of a lifelong learning journey and life of practice. Mazal tov !
In contemporary Judaism, immersion in a Mikveh is only required for women, after menstruation, before they resume marital relations.
Immersion in a Mikveh is also a component of the conversion process, wherein a non-Jew converts to Judaism, and becomes a full-fledged Jew.
Someone who was born Jewish, and does not engage in marital relations, is not required to immerse themselves in a Mikveh.
Menopausal women, who have not immersed in the Mikveh since their last period, must immerse in the Mikveh, before engaging in marital relations. Once they have immersed in the Mikveh, they most likely will never need to attend the Mikveh for the rest of their lives, unless certain Halachic issues present themselves.