Good work here, TheNiceOne.
A few suggestions:
1. To save yourself a lot of work repeating the names of .esp and .esm files, try setting vars for them initially, then referring to the shorter var name in your scripts. It also helps that if for some reason you need to adjust the names of mods, you only have to do it in one place.
Example:
SetVar ESP0 "FCOM_Convergence.esp"...InstallPlugin "%ESP0%"...LoadEarly "%ESP0%"
2. Verify the Blood&Mood / Welkynd Sword part of your script. Instead of copying the FCOM compatibility patch "FCOM_WelkyndSwordBlood&Mud.esp", it just installs "FCOM_Blood&Mud.esp" again.
3. I strongly recommend changing the optional plugin scripts to give the installer an option to install them even if the base mod is not found. The install instructions for FCOM recommend installing on a fresh install of Oblivion without any mods installed, and if users follow this instruction, many mods will not yet be installed when they install FCOM.
Example (following code can probably be cleaned up to reduce redundancy):
;** Origin of the Mages Guild **If DataFileExists "Origin of the Mages Guild.esp" CopyPlugin "=== FCOM Optional Add-Ons ===\\FCOM_OMG.esp" "FCOM_OMG.esp" SetVar OMG "1"Else If DialogYesNo "Origin of the Mages Guild was not detected. Do you plan to install it later? (Selecting "YES" will install an FCOM compatibility patch." "FCOM OMG Support?" CopyPlugin "=== FCOM Optional Add-Ons ===\\FCOM_OMG.esp" "FCOM_OMG.esp" SetVar OMG "1" ElseEndIf;** Slof's Oblivion Robe Trader **If DataFileExists "Slof's Oblivion Robe Trader.esp" CopyPlugin "=== FCOM Optional Add-Ons ===\\FCOM_SlofsRobeTrader.esp" "FCOM_SlofsRobeTrader.esp" SetVar SRT "1"Else If DialogYesNo "Slof's Robe Trader was not detected. Do you plan to install it later? (Selecting "YES" will install an FCOM compatibility patch." "FCOM Support?" CopyPlugin "=== FCOM Optional Add-Ons ===\\FCOM_SlofsRobeTrader.esp" "FCOM_SlofsRobeTrader.esp" SetVar SRT "1" ElseEndIfIf Equal %OMG% 1 If Equal %SRT% 1 Message "You indicated that you plan to install both Slof's Oblivion Robe Trader and Origin of the Mages Guild. \ %NewLine%%NewLine%\ FCOM will now install a compatibility patch for both to work together. %NewLine%%NewLine%\ Be sure you DO NOT install the OMGSRT.esp compatibility plugin separately." CopyPlugin "=== FCOM Optional Add-Ons ===\\FCOM_OMGSRT.esp" "FCOM_OMGSRT.esp" EndIfEndIf
4. One last thing. AFAIK, OBMM script command "CopyPlugin" does not activate the plugins while "InstallPlugin" does, and placing plugins in subdirectories defeats OBMM's built-in conflict detector. This has always been something of an OCD thing I do

, but moving all the .esp files into the base directory instead of an Options directory, then using OBMM script command "DontInstallAnyPlugins", and then using "InstallPlugin" for each one either required or optional just seems to make more sense to me. Of course, this would not be BAIN compatible, and users would either have to move all the optional plugins to the base directory themselves or mod authors would have to start packaging them differently.
Probably never going to happen and I'll have to keep on doing it myself, but I never miss an opportunity to influence people's thinking. :whisper:
Hope that helps.
-RMWChaos