Hi Bohémond,
I'm unsure about what happened in your test. I used your new FAC file, which I modified further (renaming Russian to New Russian). I used the Finland scenario, and in the event file added Test.sct
The script reports that everything worked as expected...

Russia called New Russian (population still named 'Russians', not 'New Russians', but that's because I forgot to change that, not a bug)
Here is the script after parsing but before execution, as you see your event is included as is, after the regular events of the scenario:
000398
000399
000400 SelectFaction = $RUS
000401 SelectRegion = $Sveaborg
000402 StartEvent = CFT2|1|0|NULL|NULL|NULL|NULL
000403
000404
000405 Conditions
000406 MinDate = 1808/01/01
000407
000408 Actions
000409
000410 SelectFaction = $RUS
000411
000412
000413 ChgFactionTag = TSA
000414
000415 EndEvent
000416
000417
Then I process the turn
Line 397: ------------------------------
Line 400: SelectFaction, selected: Russia
Line 402: Started processing event: CFT2
Line 402: This event is not yet referenced, base number of allowed occurences 1
Line 406: Min date evaluated: 1808/01/01 converted to turn 2785 current turn 2792 True
Line 408: Entering triggered actions for event CFT2
Line 410: SelectFaction, selected: Russia
Line 413: ChgFactionTag New Russian faction is now pointing to the definitions of this faction tag: TSA
Line 415: Finished processing event: CFT2
And I do get a change of Faction...