Ford A/C :: 2008 Ranger - Coolant Low / Heater Control Valve Leaking

Bought my Ranger brand new, and I try to maintain it well. Most of the 90,000 miles are highway. Anyway, the OEM coolant was supposed to be good for 100K miles, but since I hadn't done anything in 89K, i have a lengthy commute and the coolant was getting discolored, I figured that no harm would come in having a drain/refill. This was approx. The heat worked flawlessly. Fast forward two weeks, and my local shop performed an oil change (not the quick lube that did the coolant replacement). I was informed of three things:

1) my coolant was low,
2) my heater control valve was leaking (which I was shown), and
3) when hoisted I was shown that a slight residue of coolant -- not necessarily fresh -- was on my water pump.

I was told that 'down the road sometime a water pump could be in my future'. Again, the heat worked flawlessly, and I didn't want to be out-and-about in winter and run out of coolant. So, the shop replaced my heater control valve and topped off the coolant (approx. 1/2 - 1 gallon). Heat worked flawlessly. The last couple days, I noticed two things.

First, heat trickles out on low and medium setting, and second, it takes a long time to warm the vehicle. So, my limited research tells me two things. If coolant is present, and I have heat (of some sort), then, the water pump and heater core are probably fine. Diagnosis says to check the thermostat first, correct?