I have cellulose in my house. previous one had rock wool. Cellulose works better cause it forms a layer in a short period of time so the entire area is closed up. Also done by TCL - they did a good job.

BUT if you can afford it go for 200mm not just 100mm. IMHO 100mm not enough.

My ceiling is closed near 100% - all plaster ceiling holes sealed. We use a central air ventilation system from airegard for fresh air. But that is not a must but it helps if your place is permanently a/c-ed. The heat transfer you talk about will be neglible for most circumstances. But still a good idea to keep unwanted houseguests out

You should absolutely vent the space under your roof tiles or use a tiling system that breaths so disperse the hot air too. Monier has a solution for that.

1. Fresh Air, no ERV when we did it.

2. Airegard installed it - not expensive.

3. Just make sure you can access the box with ease. I swap the filter (expensive! 1xx RM per filter) once every two years.

Tip: connect to a smart switch then you can control when you cycle the air and avoid times when its too hot. I cycle when temp is below 27° and otherwise on a schedule. You could even link it to a CO2 meter and cycle once the value too high.

Same one unit can take care of the entire house for maybe 3k sqft+

Just adjust the ventilation according to what is needed - you can connect to a smart switch and if you like a CO2 monitor. then set a schedule (ideally when not too hot to save aircon money) and then set a trigger if CO2 exceeds your thrigger value to run until you are below. Last one is optional.

We use one unit with two suction and two inflow points spread out throughout the house. just make sure you factor in other suction points (toilet, kitchen) when you decide on the placement.