Correct except the part where you said that the waterpump line is return.
The water pump and or (I say and because you can try t'ing them together for more flow!) intake lines are your IN to the heater core.
The line going into the fill neck, side or whatever part of your radiator is your return.
Your best bet would be to run a line from the water pump to the heater core and T the existing manifold line into it. This will give you the best quickest flow.
i dunno, probably getting confuzzed with all this talk of return lines, feed lines in's and outs, this goes here, that goes there,,,,,whatever
i think the OP got the idea as to what can be done on how to hook up the hoses, all i know is MINE

,,, are hooked up with a hose at the back of the intake,,,
FEEDING water
TO the heater core,,,,and THEN a hose coming
OFF of the heater core feeding the water (relieved of it's heat)
TO the radiator

.
and as mentioned before, make sure your blen doors are working properly, fully closing/opening and sealing off all outside air that may be able to sneak in.
by now, with a hotter t-stat, running hoses correctly, and if the blend doors are working properly, you SHOULD have heat. (i would think

)