Is it helpful (or harmful) to have a letter of recommendation from an alumni?
Does the person know you? Is the person going to say anything important that is not said anywhere else in your app?
You mean separate from the alumni interview?
If it is someone who knows you really well – a teacher, boss, coach – sure, submit it. If it’s just a family friend, a neighbor, your mom’s coworker – the chances that it will make a difference are really small.
How much do they donate annually?
Unless it happens to be a person who was already going to write a LOR for you, it’s harmful in that it waters down the letters that really matter, those from your teachers. Don’t do it.