Hello and welcome.

1) How important do you feel honesty is in BDSM relationships?
Be honest at all times and you will have trust and not be likely to be thought of as cheating .

2) How important do you feel trust is in BDSM relationships?
Trust is a earned thing. It's built by honest and actions. Not all actions are obvious.. it's the cumulative "score" that grants you the trust. You don't get it as a Dom by walking up to a sub and start giving orders.

3) What constitues cheating in a BDSM relationship?
two kinds of cheating i see :
the kind between two or more people that has not been discussed prior and often should be discussed before the relationship if possible so all is always open and honest.'
Then there is the cheating ( deceit and/or deviousness) as in portraying yourself as someone whom you are not. whether online or real life.. eventually it catches up with you .
Advice is to be yourself .. Don't lead others into thinking you are more experienced than you are.. or that you are more willing than you actually are.
Life is hard enough why make it more difficult and/or more complex.