Yes, Christianity does start with the heart. Do you know if buddhism even acknowledges the heart?
Again, not a Christian perspective. Are you sure about this?
Dress rehearsals are real. They are a preliminary to find and correct mistakes. They ARE imperative and quite neccessay. Yes, life is very real and not a sideshow.
Yes, it does. In the Mahayana tradition the bodisattva refuses to enter nirvana untill all are liberated, so we have the idea of sacrifice for others.
Susan Salzberg has a wonderful book on Loving Kindness meditations
The way I learned it the prayer goes:
May you be filled with loving kindness
may you be well
may you be peaceful and at ease
may you be happy.
In practice we start by visualizing a "neutral person" like a stranger we see or meet and say the prayer for them
Then we move to saying the prayer for someone we are in conflict with an enemy if you will.
Jack Kornfield in his book A Path With heart outlines much more about the buddhist idea of "heart" as well.