Urban heat island effect is cause by cutting down too many trees (which reflect infrared) and paving over the majority of the surface. Here too mechanical heat is dwarfed by sunlight.
I'm not so sure, I can't find numbers but it is commonly listed as a secondary factor (including by wikipedia).
I did find this: "Results show that the urban heat island causes an average increase of 2.2 °C in the external air temperature mainly caused by the waste heat rejected from cooling system" (https://www.mdpi.com/2225-1154/9/3/48/htm)
And while it's true UHIs don't significantly contribute towards climate change, they can cause increased rainfall, and presumably have some effect on agricultural production that's done near larger cities.
You underestimate how wasteful suburban life is. If you have 2,000 people in a square kilometer all burning tens of kW of petrol in their cars and running multi kW AC systems in their homes and their office as well as pools, giant LED billboards and whatever else it starts to give sunlight a run for its money.
