1/ As a general rule, it is probably best to let the oven stabilise at the desired temperature before putting the blade in. The main reason I can see for doing this, is not that any particular sequence is technically "better" than any other sequence, assuming you always follow the same sequence and optimise it, but that it is a PITA if your optimised sequence involves starting from a cold oven. It will take several hours to cool from Austenitizing temperature, so starting from cold each time will mean you can only HT one batch of blades a day.
2/ SS foil is usually used for air-hardening steels. These will harden with a relatively slow quench and the use of quench plates with the foil still in place provides fast enough cooling to harden them. The quench plates are there to minimise distortion as much as to provide fast cooling. It is worth understanding that stainless steels are stainless because they are MORE reactive than Carbon steels. Oversimplifying slightly, the Chromium in the stainless readily forms an Oxide which takes up the same volume as the base metal underneath and seals against further attack by Oxygen. If the Oxide layer gets scratched, a new Oxide layer will form very quickly on the exposed metal and the Oxide "skin" is self-repairing. At high temperatures, the Oxide layer on a stainless steel is not able to keep the Oxygen from reaching the metal beneath anything like as effectively as at normal temperatures and the Oxide layer gets thicker. The simplest way to deal with Oxidation of stainless steels is to prevent it by excluding Oxygen, which is what the foil packet does.
Iron and Carbon steels form Oxides which take up more space than the original metal, so they bunch/break up and allow further Oxidation to take place underneath.
With Carbon steels, which are usually water- or oil-hardening, the required quench speed is usually too fast for a foil packet: the foil is a stainless steel, which is a fairly poor conductor of heat as metals go, and there will be a thin layer of air between the foil and the blade, which is a further insulator.
Carbon steels are usually Austenitized bare, or with a thin coating of an anti-scale compound. The anti-scale compound excludes air from the surface, but is a thin layer in intimate contact with the surface and does not slow the cooling significantly when the blade is quenched in oil or water.
3/ Zero degrees. In part this is down to your process and your mindset. If someone else's process says 1575 degrees and you find your process works better at 1580 degrees, you can either view it as an extra 5 degrees over "the" temperature to allow for differences between their system and yours, or you can consider your target temperature, with your kit, to be 1580 degrees.
4/ TBH I don't know. As cool as possible is the realistic answer. Use the biggest heatsink you reasonably can, use the best heatsink paste you can get (I used Arctic Silver on most of my early ones because the computer nerds seemed even more anal about heat transfer than I am and it's what they seemed to use on CPU heatsinks). Ensure good airflow. A metal enclosure usually gets rid of heat better than a plastic one. Keep a spare SSR in stock
5/ "Long enough". It's really not helpful, I know. Between 10 and 20 minutes after the setpoint is reached seems reasonable with most of the HT ovens I've built and/or seen, but you'll need to watch what happens with yours: You want to get a consistently repeatable procedure with your equipment. What other people do with different equipment is not something you should worry about overmuch.
