Cooling details

1.) For every reactor tick, each component is processed once, one after the other. It starts at the top left, the progresses horizontally until the top right. Then returns left one row below and progresses horizontally again. Every component processed gets to do its thing, whatever that may be - drawing heat from the reactor hull or passing it there, spreading heat among itself and/or adjacent components, cooling itself, cooling adjacent components, generating heat and/or EU. That means the order of components plays a role, and also the reason why no two components in a symmetrical design have the same heat state. One of them simply gets to act before the other.

2.) If you remove the middle bottom cell, you remove a 3-neighbor heat source, drop another 3-neighbor down to 2 and a pair of 2-neighbors down to 1. That's a massive reduction in heat generated, and thus several of the cooling components simply have nothing to do anymore. When their turn comes, there is no heat left to dissipate/spread/transfer etc because the components further up who get to act first already took care of everything.

3.) Correct

4.) The component heat exchanger balances heat level between itself and surrounding components. If you remove the top right vent, the other two vents adjacent to the exchanger start overloading themselves due to the reactor now generating more heat than there is cooling capacity for. And the exchanger will dutifully balance itself with them. It will make sure its own heat% is as similar as possible to that of the vents, which are making a beeline for 100%. Thus the exchanger is also making a beeline for 100%. But because the vents melt first and the simulation stops there, it doesn't quite go all the way.

5.) No, each OC vent draws the full 36, as long as there is heat available to draw. That's why you have the two heat exchangers. They balance the heat level between the five upper OC vents so that all of them are getting an equal share, even though some of them will draw more than they can handle and some of them will have nothing to draw.

6.) The two bottom OC vents incidentally receive heat directly from the adjacent uranium cell. A fuel cell will divide its heat output between all possible adjacent components that can receive heat, if there is at least one. Now, almost all reactor designs place their fuel cells so that they are not adjacent to anything that can accept heat; in that case (and only that case), they dump all their heat to the hull. The component vent in the third-from-bottom middle slot cannot accept heat for example, and neither can other uranium cells. The OC vents however can, and because they're the only adjacent component that can, they get all of it. All 24 that the corner uranium cell in question generates. As it happens, their native cooling rate is 20, and the component vent adds 4, so the 24 heat they receive is exactly compensated. Also, because they are so far down in the reactor, there is no heat left to draw from the hull when their turn comes, and thus they don't melt themselves.

7.) Also unsure as to why that is, but it's an effect you can observe in almost all reactors. Most components will keep a baseline of a few dozen heat points before the adjacent exchangers and such start cascading the heat away. It depends on how many steps the heat must move, because there needs to be a small differential at every step. It's not really important though, you'll find that the components stabilize themselves in a certain state eventually and, after switching off the reactor, cool back down within a couple seconds at most.

7a.) Heat not dissipated or transferred away adds up every tick, yes. But as said above, sometimes you first need a certain treshold before the transferring away really starts working properly.