How do I fix my heat?Heat not blowing/coolant leak from hose near coolant tank

What is your question?

I have been dealing with some ongoing heat issues with my 96 cadillac eldorado and need some help. I recently took it to a local mechanic to have the heat fixed. Prior to taking the vehicle to the shop I had hot air coming out very slowly. At higher speeds(60 mph +) hot air would come out more steadily but as soon as I slowed down....so did the air flow. I also have had an ongoing coolant leak. The coolant would leak out from a hose near the coolant resevoir and would only stay about 1/3 of the way full (in the coolant tank) most of the time. When I filled the coolant to the top it would leak out immediatly. This leak did not effect the temp of the car and the overheating was not an issue. The mechanic originally told me it was just a hose that needed to be fixed. The hose was fixed and when the did a pressure test it blew "some metal heating rods", Which I then paid to be replaced. When I picked up the car the heat was working but on the dash it said "check coolant level"- this had never shown up before and I told the mechanic...he said it was a sensor...I was so pissed by this point(this was not the only issue with the work I had done)....that I left. The next day I noticed the coolant was a little low but not much. The heat worked fine for about 2 weeks and then it stopped. Now I have the same issue I had before...the coolant leaks till its about 3/4 empty....no overheating issue....hot air does comes out at high speeds....but not at low speeds or when the car is at rest. The mechanic says its either the thermostat or a blower motor...the blower motor is newer....and the car doesnt overheat... My main concern is getting this fixed once and for all without being swindelled and paying too much or fixing things that dont need to be fixed. in addition the "service emmisions system" and "service transmission" warnings come on from time to time....I had a sensor replaced for the transmission(or so they tell me) and the service engine light goes on for awhile (not flashing) and then goes off....not sure if this is related or not..... Any Suggestions?