First - You're sure that there is no blockage in that kickspace radiator? Could be whole problem.
Googling "what is a monoflow tee" and got these results, which may help
"One-pipe system with monoflow tees.
"Monoflow tees are used to divert some water from the main loop into each radiator. Use one monoflow tee on the return if the radiator(s) are located above the main loop; use two monoflow tees if the radiators are installed below the main loop. Place the tees in the main loop to each radiator at least 12" apart."
I'm not a plumber, but, from your sketch, if you want more heat from that kickspace heater, and the specialized tees don't work, you could put (say) a ball valve on the bypass pipe to the heater -- the pipe connecting the standard and monoflow tee -- and reduce it's flow. Depending on amount of restriction, more, or less hot water will flow through the that kickspace radiator. Just my kludge fix.