Problem with visual and html editor

Hi!
My problem is as follows : I can’t access the visual editor at all anymore. I don’t care because I didn’t use it, but I have several other users on my blog and they don’t really know html, so they really need to be able to use it.
First of all, I spend the last 3 hours searching the forum, google, trying various hacks and solutions, and nothing worked.
I desactivated all the plugins, didn’t change anything. I switched to the default theme, same. I used the .htaccess hack (I had a former problem with the flash uploader two months ago, I solved it with the no flash uploader as nothing else worked. Of course, the first thing I did was desactivate the plugin, even though I used the blog several times since the installation and everything worked perfectly. But I tried the .htaccess hack anyway, just in case), I cleared the browser’s cache multiple times, I also cleared the js cache containing the tinymce data, I tried three different browsers on two different computers (firefox, ie and safari, first two on PC with win xp, firefox and safari on mac with Leopard. I could try firefox on linux, but I don’t really see the point…), I tried login in with an other account (I have admin and basic accounts), I tried unchecking the “use the visual editor”, then checking it again, I looked in the sql database (richeditor was set as “true”), I tried the compress = false hack in the tinymce file that I found on this forum…
Nothing works.
There are two things I haven’t tried and I will not try : doing a fresh install, and upgrading the WP version. I know some of the plugins I’m using right now and I really need would not work with a newer version, and I had to modify some files to make them work with the plugins, I don’t want to spend the next week trying to find which ones were modified and which were not.
Anyway, this WP blog has been working flawlessly for the past two years, appart from the flash uploader that suddenly refused to work two months ago, and I didn’t touch a single thing, so I don’t understand what could have gone wrong.
I noticed, using the debug console, that quite a lot of functions were “undefined”, and variables were undefined as well, some of them associated to switcheditors, editor.js, tinymce. But I really don’t know what to do with these files…

Any clues on the next steps to take, here ?

Thanks a lot !

(and merry christmas ! Yes, I know, it’s christmas, and I’ve been spending all morning trying to fix this issue… I really need it to work !)