What is the minimum number of modifications in order to install a custom ROM? I am planning to upgrade my wife's Galaxy Ace II from 4.1.2 (the latest official firmware offered by Samsung, installed via Kies)  to 4.4.4, which is available via http://novafusion.pl/downloads/ as an unofficial CyanogenMod 11.0 variant.  I have tried to gather as much information as possible, but some things are not clear yet.  I am seeking short, precise, technical answers; there are enough "10 steps for ... your phone" out there already. ;-)


I know what rooting means, but apparently it is not always necessary in order to install a custom ROM? (This seems to differ from Jailbreaks on iPhones.)
However, I probably have to "unlock the bootloader" in order to make the phone accept a firmware that is not signed by Samsung? (I understood that this is what "unlocking" the bootloader means.)
If I understood correctly, the (/a) recovery mode is always available and mainly offers the features to a) wipe the data partition, b) wipe a cache partition (which seems to be hidden from the UI, so I did not know about it before), or c) install an update.zip from an SD card.  Apparently, there are more advanced custom recovery ROMs/firmwares out there, too, which is probably analogous to installing Grub instead of Windows' default bootloader.  But do I need no install a custom recovery ROM, too?  (Why) Is that recommended?
To install a rom you have to root your device, there is no way without rooting your phone as long it isn't rooted yet
Sorry dont understand your second question. 
You have to flash a custom recovery such as Clockwork Mod Recovery. You need a custom recovery because you can't control the default recovery (no way to acces it). So you have to set your phone in downloadmode and flash the root and the recovery. 


Have a loook at this step by step  tutorial where everything is explained ( http://forum.xda-developers.com/showthread.php?t=2215996).