Update on my last post about my broken shifter: I tried the sanding method, as I initially suspected that the plastic parts on my new shifter expanded due to heat (I live in a tropical country). Sanding did not resolve the issue, and I may have broken the shifter, as it will not shift anymore.
I ordered a Sturmey Archer DLS30 shifter, removed the protruding part with the gear indicator, and placed it under the brake levers; my bike now shifts flawlessly! The catch is there is no gear indicator anymore, but it is not a big deal for me.
I hate how flimsy the Brompton shifters are, given the bike's expensive price. Good thing the Sturmey Archer shifters only cost $10.
by fuegolds