Tell us what state you are in, and probably we can get better input from someone in your area. I am in S. Calif. Here we do not see boilers in residential heat, and it would make no sense to have domestic hot water served by a heating system that we really only want to run 2 months a year, in a cold year! Even if you are in a cold climate, it makes sense to me for domestic hot water to be separate from the heating boiler, but I am not up on the economics of those kinds of systems, so lets hear from the other guys on that.
The issue of not enough hot for all the apartments will probably be addressed by the new boiler or whatever.
The one apartment with low hot water flow, first check the valves or cartridges in the shower itself. Then look for a hot supply isolation valve for just that apartment which may be partly closed, or may have become clogged.