You probably have more info than you ever wanted to know after searching, but I thought my experiences may help in some way.
I used (still have) a Charbroil offset smoker and after a Google search it looked similar to your smoker. My grill has a pretty small firebox and also leaked a good deal of smoke. I found the biggest issue I had was my temps would get hot and then after a while level, very briefly in a range I wanted to smoke, and then would drop and I would fight to keep them up. I always used the minion method and kept adding coals every so often to keep the heat going.
The easiest fix is usually the obvious one, so make sure your vent on the firebox was open and letting air get to the coals. For the not so obvious maybe - I don't know about your particular model, but for me, the size of the firebox was really impeding on my ability for longer smokes. In the beginning of the smoke I would be OK, however, as time went on and I needed to add coals the ash would build up so much it was essentially snuffing out my fire and I wasn't getting enough air in the firebox. Unfortunately, I did not have the ability to open a side door in the box, only the lid, so I didn't really have a way to dispose of ash. I often used a metal skewer like a fire poker to get air going through the coals, or I would lean the coal grate up so more air could get to the underside of the coals. You had already mentioned using lump to cut down on ash, so it seems like you air having the same problems I had. Another thing that may help is add coals that are already grayed over - maybe that would help keep your heat up and cut down on some ash (just an idea).
Also, try not to get too overwhelmed by trying to keep the temp regulated and what not (not saying you are, I just know I would get bothered by it). Learning your smoker and what works and what doesn't is part of the fun and challenge of smoking meat. I actually look at it like a badge of honor to control the temp. using wood or coal while battling the elements.
Sorry for the long rant, maybe something in this is of some use. I hope you get your mods and things working the way you want. Also, glad you enjoyed the ribs ! Eating the results can be the best part!