Competing conditioned/combustion air flow; worry about CO

I am a little concerned that you are overthinking this situation. Naill C gave you some good advise in your last question. There are very few houses not built to be virtually air tight with an air exchange system that do not have enough natural air flow to support open combustion appliances.The simple act of opening a door to enter or exit the house allows large amounts of fresh air in during cold weather when windows are closed. Unless you have made extreme attempt to block all air infiltration, there is probably pleanty of combustion air passing through your louvered/slat door. I totally understand your desire to minimize heat loss from the rest of the house. Before any external venting will improve the situation, the utility room will have to be somewhat sealed from air flow from the living space of the house. In new construction a vent of a calculated size is brought in from the out of doors, screened and baffled, and strategically located just above the floor by the appliance. The mechanical room is almost air tight, insulated and lined in a fire rated material. Since this space is indoors and only calculated amount of outside air is allowed in, freezing is not normally a concern if water utilities are also in the room. Drawing air from an attic can be risky. Obviously some think it a good idea, however there are drawbacks. If the attic is not well ventilated the air may not be sufficient. I simple vent register in the ceiling to the attic will let as much heat up as it pulls air down. In fact, any open path into the attic can be dangerous. A register, vent pipe etc. can act as an easy path or chimney for fire or natural gas to spread quickly into the attic. If you decide to proceed in isolating and venting your open combustion appliances, pick one of the proven techniques and run it by your local building code officer for local compliance. Better safe than sorry.

First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.

I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.

Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.

Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.

This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.

One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.