Is it a bad idea to tie output of hot water heater to sump pump drain?
Do not connect the overflow from the water heater to the a/c condensate line as you have marked. If the TPV ever did open wide, then water would flow back into your a/c evaporator and gas-fired furnace. This is not permitted and would be very destructive.

You could direct the output of the WH to the same pipe that the a/c condensate goes into by using the proper fittings, but if I were you I would direct my efforts to determining why the TP relief valve is leaking. What temperature do you have the WH set at? Is the water coming out of the hot water taps extremely hot?

Then I would attach an expansion adapter near the floor to the end of the pipe the condensate goes into. This would allow both pipes to feed into the pipe going to the sump and provide a visible air gap between both pipes and the pipe leading to the sump.