Your questions are quite broad and there aren't really specific answers to them.
Your question #1 has no general answer. It just depends on what kinds of investments are in the trust.
As for your question #2, it's not really possible to guarantee both intact principal and unvarying revenue.
That can be approximated by setting some level of expected return, and then paying out on a schedule which is calculated, over time, to cancel out that return, leaving the principal intact (or perhaps allowing the principal to grow with inflation). But if the actual returns are less than the expected returns for an extended time, the principal may decrease. Also, if the actual returns are more than the expected returns for a long period of time, the beneficiary may pressure the trust administrators to increase payouts, and if they do, the payouts may get too big.
So there are no guarantees either way. There is no magical exact way to determine "how long" is too long for the payouts to be "too big" before the payment schedule should be adjusted; the trustees/managers just have to pay attention and try to set a level that is sustainable. In some cases, the trust terms may specify in detail how the money is to be allocated and managed, while in other cases trustees have more latitude.