Leftotian

What proof? Just go play the game and see for yourself.Test stuff right on the runway to save time. For example make large ISRU + 8 large drills. Run two channels, lf and ox(if you run more than 2 it will overheat no matter what, it can only take 500 and you need 200/channel).Test with retractible, then with fixed. Just don't forget to put ISRU/drills and their radiators on different parts. Tell me how much radiators you need to run it all without overheating. I can tell you right now how much, but try to figure out yourself. I can tell you that you will need 3x kW in retractibles, which means ~15x more weight.

Literally every single thing you said is wrong. There is a bug. Retractible radiators are unreliable because of the bug. The fact that they work no matter where you place them is what causing the bug and the very reason why you should not use them. Fixed radiators do not rely on orientation of the sun, so you need to use the exact amount, while you need to use 200-300-400% more retractible radiators to compensate for the bug, which, combined with the fact, that they are 5 times heavier than fixed ones of same power, makes them weight 10 times more for same amount of cooling. The fact that you need to put fixed radiators on same part as coolant consumer is an advantage, that makes them way better than retractible radiators. Exercise in futility is using 4 tons of retractible radiators to coold down a craft, that will function just as well with 300 kg of fixed ones.

Do you actually want to hover or do you just need to lift stuff? The easiest thing to do is to make long(so your docking port is just a bit higher than the target) legs with aircraft landing gear at the bottom, land anywhere near, drive on top of it(use rcs to push your craft around) and lower your gear to dock.

Small ISRU needs 100kW/converter(up to 400), but "max cooling 50kW" so no matter how many radiators you use it will only get 50kW. That means that small ISRU can't not overheat. Just use one 10kg 50kW radiator directly attached to ISRU for max performance, you can't make it run better than that. Never use retractable radiators, they are bad and broken, which makes them twice as bad. For example, if you are trying to run full power large ISRU + 4 large drills you will need 2500kW = 2.5t of retractable radiators or just 900kW = 220 kg of surface attach radiators. Actually all radiators are broken(actually actually, drills are broken, but whatever), but with surface attach ones you can avoid the bug. You need to make sure that drill radiators don't interact with ISRU and vice versa, so make sure to put your drills/ISRU on different craft parts and keep your drills/ISRU radiators separated.

The answer to "WHY?" is: bugs, as usual. Nobody really designed it like this. ISRU doesn't have this limitations by design. Production rate dependent on ore storage comes from low refresh rate of distant vessels. Low refresh rate is there to improve bad performance of the game not to lower converter output. It's an unintended consequence of two unrelated systems not working properly together, basicly a bug.

For a day. It actually updades at the start of the day, so it never really stops, but it will only convert as much as can fit in your ore tanks per day. So if you need to make 3000 fuel and have 1500 ore tank, it will take you 2 days, but if you have only 75 ore tank, you'll need 40 days for same result. If you are controlling your craft same work will only take 30 minutes(if you have enough drills to keep up with ISRU).

Drills need 100, but they consume as much as you will give them, so if your ISRU eats 800, drills will also eat 800, you'll need 2400 to run it all at 100%. It's bad example, because 2 drills will not allow you to run ISRU at full power. Better example from practice: i have ISRU running lf, ox and lf+ox, for 600 kW. I need 16 drills to feed it(7+% ore, lvl3 engi), that's another 16x600 = 9600 kW. So i need 10200 kW worth of radiators to run all of that stuff. It should be 600+16x100=2200, but it's 10200 instead. It's a bug, i know, but that's how the game works right now, so you need way more radiators than what tooltips suggests.

Not sure if you still need an answer month later, but since noone answered about overheating: ISRU needs 200 kW/activated convertor, so up to 800 kW cooling. Every drill will eat as much as ISRU, so if you are running all 4 converters on isru + 2 drills you will need 2400 kW of cooling, if you are running 1 converter + 2 drills 600 will be enough.