It sounds to me that it has been poorly installed from day one
point 1 probably due to incorrect pipe configoration, causing air to be drawn into the system
point 2 again incorrect pipe configoration causing reverse circulation
point 3 probably due to no valve on HW primaries, hot water is taking priority
You definately need a two port valve on the hot water you cannot control the hot water without one
you would be wise to call in a qualified engineer to survey the system and give his oppinion and remedy suggestion, get at least three different quotes and use reputable engineers try the Institute of Plumbing
www.plumbers.org.uk you can then use the search facility to locate local engineers