Can't follow your post BUT:
1. Turn signal switch has nothing to do with the headlights or dimmer.
2. The whole thing is simple. Regardless of terminal strip or not, the wiring is per below however you get there.
Dimmer has 3 terminals
One for hot wire from headlight switch (center terminal)
One for Hi beam
One for Lo beam
Does not matter which one you chose for Hi and Lo
The headlight bulb has three terminals
One for ground
One for Hi
One for Lo
The bulb grounds can be a separate wire to the body/frame or can be jumpered together as one ground to body/frame.
The Hi and Lo wires from the dimmer usually go to the left light and jumper to the right side.
The Hi wire, from the dimmer, also gets a wire spliced to it that goes to the dash bulb.
Hard to figure your problem out from afar, you will have to solve it by checking the above hook up and be SURE your grounds are straight from the bulb to the body or frame. "Plenty of ground" sounds like you are too sure. You can't blow it off like that. YOU HAVE TO BE SURE. Please keep in mind, most troubles and troubles like yours, are the ground path from the bulb to battery negative. A tortuous path indeed because it's not just a wire like the hot's are.
Also check, as said, those silly terminal blocks that should be eliminated. And check all the wiring. Check that the Headlight switch body is grounded to dash.
When it's all rechecked three times, when all grounds are ACTUALLY good, terminal blocks checked, dimmer wiring and headlight switch wiring and bulb sockets/wiring are checked...it will all work. So dig thru it all and clean it up.
beware of old/sagging/broken terminal strips, old brittle bakelite headlight bulb connectors, bad headlight switch, bad dimmer, loose crimps, mis-wire, loose screws, loose contacts, corrosion, weak battery and possible need for a relay.
Good luck and report back.
