Plugin.js is essentially the parameters underwhich the game loads, such as (And exactly): <param name="cabbase" value="loader650091953.cab">
<param name="worldid" value="34">
<param name="members" value="1">
<param name="modewhat" value="0">
<param name="modewhere" value="0">
<param name="lowmem" value="1">
<param name="lang" value="0">
<param name="plug" value="0">
<param name="js" value="0">.
The World ID only affects the number at the bottom right of the logon screen; it doesn't actually change the world. Cab base locates the .cab file from which it loads (Which I assume you Decompiled and deobfusicated to get the rest of the files, like the js5)
Members determines whether the names of the "member's objects" that you are in posession of are named their true name or just the same old "member's object". Every other member feature is left untouched, sadly.
Modewhat and Modewhere are in my current research.
Lowmem is for High Detail or Low Detail
Plug I've not experimented with yet.
Js I've not experimented with yet.
I tried decompiling "runescape_958005245.js5" with DJ Java Decompiler, but it was to no avail, as it just became a big bowl of text puke.
Thanks for the tip on the pack200 by the way. I've spent the past few minutes trying to figure out how to work unpack200.exe in the java folder.

I've noticed several connections between the Jagex Cache on my computer and the various loader files that you listed, but I've not been able to clearly visualize them. I'm not, in the least bit, skilled in Java, nor do I know anything about it.
If anyone wants my files that I've been experimenting with, just ask for them. I'll probably winrar them and upload them to my crappy host, but it sounds like martinmine and I are at the same spot, stuck on runescape.js5.
Martinmine are you still working with .class files or did you decomp them into jads?