The only reason to use the internal blocks like
ACTION_IF FILE_EXISTS_IN_GAME ~fw100.are~ THEN BEGIN
set_up_tutu_stuff_using_underline
END ELSE BEGIN
ACTION_IF FILE_EXISTS_IN_GAME ~ar7200.are~ THEN BEGIN
set_up_bgt_stuff_using_underline
END ELSE BEGIN
FAIL ~This is not Tutu or BGT~
END
is if there are things that need special handling in only one of the two game environments. It doesn't look like there is anything here that needs special handling, so a straightforward %tutu_var% on those items should work fine, as tejumin says.
You will need to look at the bg1npc\lib\g3_bgt_cmpvars.tph and bg1npc\lib\g3_tutu_cmpvars.tph files - stuff like
WRITE_ASCII 0x250 ~_HOUTDLG~ #8
WRITE_ASCII 0x268 ~_TASIGHT~ #8
ends up with new definitions, so it looks like
WRITE_EVALUATED_ASCII 0x250 ~%tutu_scripts%HOUTDLG~ #8
WRITE_EVALUATED_ASCII 0x268 ~%tutu_scriptw%TASIGHT~ #8
{ SHOUTDLG WTASIGHT}
I don't think absolutely everything in BG1NPC has been optimized completely, even though that was the test mod for the whole crossplatform variable setup, so I would look at Ascension64's stuff