Q: Will a medium fit a combo lock?
A: The saddle bag can fit a combo lock. However, weight can play a factor in the fit since too much weight might pull the bag down interfering with the tire.
Q: Will the medium fit a 7" bike pump?
A: If we assume that you're talking about a mini pump, the type that's typically frame mounted; it should fit just fine. Those pump designs are usually pretty narrow with handles that are about the same width or collapse down to the same width, so there shouldn't be any issues.
Q: How sturdy is the Velcro fastener that fastens under the seat?
A: Holds very well and only moves slightly
Q: How much does it weigh?
A: About 5 to 6 ounces. It's nice and lite.