1) Currently: first time you call ezini for a specific ini file, it will check for the existence of this file with all four variants (ini, ini.php, ini.append and ini.append.php ) in all possible locations in all activated extensions, when it is done it will create cache file name based on this array of file paths and then compare last modifed time on all those files compares to the cache file.
Cache is read or generated and put into ini instance's local variable cache. The next time the file is requested (during the same page request) it is read from local variable cache.
For a somewhat more performance related approach ('ezini_performance_full_pluss_devmode.diff'): http://issues.ez.no/IssueView.php?Id=12137&activeItem=24
2) good question, just added it as operator in OE my self ('ezoe_ini_section')
3) see 1)
4) Depends, earlier ini settings where compiled into the template when the template where compiled (unless you specified the dynamic flag), but I'm not sure it does that in trunk right now.
I've got another question concerning ini files : Why eZSystems choose to put some reference data such as location data in an ini file rather than in a table ?
