All right, so I spent my day charging through FG quests to test out what's going on with our little fetcher. 

 Here's what I've got. On killing Maglir, I 
was expelled from the guild, although I got no quest pop-up saying as much. After Maglir was killed the Fighters' Guild was no longer listed on my faction screen, and speaking to any member of the guild after that results in the "you killed a member; go speak to Vilena Donton" speech.
Using FormID on Maglir during "More Unfinished Business" returns the following, faction-wise:
Faction: Blackwood Company - 0002FF29 - Faction is EvilFaction: Skingrad Citizen - 00028E77 - Faction is HiddenFaction: Leyawiin Faction - 000338EC - Faction is HiddenFaction: Chorrol Citizen - 00028D98 - Faction is HiddenFaction: Bravil Faction - 0002CA32 - Faction is Hidden
SameFactionAsPC returns 0.00. None of my other mods seem to be modifying him, as FormID confirms he's from Oblivion.esm, as are all the factions he's a part of.
... and now, during "The Hist," getting this info as he's charging at my character's face:
Faction: Fighters Guild - 0002228FMod: 00: Oblivion.esmFaction: Blackwood Company - 0002FF29 - Faction is EvilMod: 00: Oblivion.esm
SameFactionAsPC also returns 1.00. So, something's added him back into the Fighters' Guild faction when he obviously shouldn't be.
And FWIW, my modlist:
	Spoiler 	Masters for: Save 26 - Amarel Mathmeldi - Modryn Oreyn's House, Level 27, Playing Time 171.32.06.ess00  Oblivion.esm01  EnhancedWeather.esm  [Version 1.4.1]02  Cobl Main.esm  [Version 1.72]03  Kvatch Rebuilt.esm04  Unofficial Oblivion Patch.esp  [Version 3.3]05  Oblivion Citadel Door Fix.esp06  DLCShiveringIsles.esp07  Unofficial Shivering Isles Patch.esp  [Version 1.4.0]08  EnhancedWeather.esp  [Version 1.3.5]09  Enhanced Weather - Across-The-Borders Mods Patch.esp  [Version Final]0A  All Natural - Real Lights.esp  [Version 0.9.9.5]++  Advanced_Water_Modification-2734.esp0B  Atmospheric Oblivion.esp0C  Distant Chapel Bells.esp0D  WindowLightingSystem.esp0E  SoT_Holiday.esp0F  Enhanced Economy.esp  [Version 4.2]10  Crowded Roads Revisited.esp  [Version 1.1]11  FF_Real_Thirst.esp12  Map Marker Overhaul.esp  [Version 3.5.1]13  Map Marker Overhaul - SI additions.esp  [Version 3.5]14  DLCHorseArmor.esp15  DLCHorseArmor - Unofficial Patch.esp  [Version 1.0.5]16  DLCOrrery.esp17  DLCOrrery - Unofficial Patch.esp  [Version 1.0.3]18  DLCVileLair.esp19  DLCVileLair - Unofficial Patch.esp  [Version 1.0.5]1A  DLCMehrunesRazor.esp1B  DLCMehrunesRazor - Unofficial Patch.esp  [Version 1.0.4]1C  DLCSpellTomes.esp1D  DLCSpellTomes - Unofficial Patch.esp  [Version 1.0.1]1E  MaleBodyReplacerV4.esp1F  TFF_Races_Base_RSM.esp20  Slof's Male Clothing.esp21  UFF Armor of Gaia.esp22  TFF_Armor_Base.esp23  TFF_Clothing_Base.esp24  TFF_Clothing_SI.esp25  Accompt of the Delitescent.esp26  UFF & RM Assassin Elite Armor.esp27  Pirate Gear.esp28  UFF - Celtic Witch Robes.esp  [Version 2.1]29  UFF RM Ranger Armor.esp2A  Slof's Horses Base.esp2B  Slof's Extra Horses.esp2C  DLCThievesDen.esp2D  DLCThievesDen - Unofficial Patch.esp  [Version 1.0.5]2E  DLCThievesDen - Unofficial Patch - SSSB.esp  [Version 1.0.4]2F  Cobl Glue.esp  [Version 1.72]30  Cobl Si.esp  [Version 1.63]31  FF_Real_Thirst, Cobl.esp++  Cobl Tweaks.esp  [Version 1.44]32  Ivellon.esp  [Version 1.8]33  Better Benirus Manor for Cobl - Complete.esp34  BHC_Expanded.esp  [Version 1.2]35  hackdirtabyss.esp36  ImpeREAL City Unique Districts - All the Districts - Merged.esp37  Kragenir's Death Quest.esp38  KDQ - Rural Line Additions.esp39  Kvatch Rebuilt.esp3A  Kvatch Rebuilt Weather Patch.esp3B  Slof's Oblivion Goth Shop.esp3C  The Ayleid Steps.esp  [Version 3.4]3D  UFF Gypsy Market.esp3E  Cyrodiil Travel Services.esp  [Version 2.0.3]3F  DLCBattlehornCastle.esp40  DLCBattlehornCastle - Unofficial Patch.esp  [Version 1.0.4]41  DLCFrostcrag.esp42  DLCFrostcrag - Unofficial Patch.esp  [Version 1.0.4]43  Knights.esp44  Knights - Unofficial Patch.esp  [Version 1.0.9]45  SM Plugin Refurbish(Merged).esp  [Version 1.30]46  TOTF.esp++  TOTF Delayer.esp47  RTT.esp++  Enhanced Weather - Ruined Tail's Tale Patch.esp  [Version Final]48  Blood&Mud.esp49  Blood&Mud - EE patch.esp  [Version 4.2]4A  Shadowcrest_Vineyard_COBL.esp4B  Shadowcrest_Vineyard_GuardCOBL.esp4C  xuldarkforest.esp  [Version 1.0.5]4D  xulStendarrValley.esp  [Version 1.2.2]4E  xulTheHeath.esp4F  XulEntiusGorge.esp50  xulFallenleafEverglade.esp  [Version 1.3.1]51  xulColovianHighlands_EV.esp  [Version 1.2.1]52  xulChorrolHinterland.esp  [Version 1.2.2]53  xulBeachesOfCyrodiilLostCoast.esp  [Version 1.6.4]54  KragenirsDeathQuest-LostCoast patch.esp55  xulBravilBarrowfields.esp  [Version 1.3.2]56  xulLushWoodlands.esp  [Version 1.3]57  xulAncientYews.esp  [Version 1.4.3]58  xulAncientRedwoods.esp  [Version 1.6]59  xulCloudtopMountains.esp  [Version 1.0.3]5A  KragenirsDeathQuest-CloudtopMountains patch.esp5B  xulArriusCreek.esp  [Version 1.1.3]5C  xulPatch_AY_AC.esp  [Version 1.1]5D  xulRollingHills_EV_withoutWheat.esp  [Version 1.3.2]5E  KragenirsDeathQuest-RollingHills patch.esp5F  xulPantherRiver.esp60  Blood&Mud-PantherRiver patch.esp61  xulRiverEthe.esp  [Version 1.0.2]62  xulBrenaRiverRavine.esp  [Version 1.0.2]63  xulImperialIsle.esp  [Version 1.6.5]64  xulBlackwoodForest.esp  [Version 1.1.0]65  xulCheydinhalFalls.esp  [Version 1.0.1]66  Blood&Mud-CheydinhalFalls patch.esp67  TearsOfTheFiend-CheydinhalFalls patch.esp68  KvatchRebuilt-CheydinhalFalls patch.esp69  DungeonsOfIvellon-CheydinhalFalls patch.esp6A  xulAspenWood.esp  [Version 1.0.1]6B  xulSkingradOutskirts.esp  [Version 1.0.1]6C  BarrowFields-Blood&Mud-Patch.esp  [Version 1.3]6D  AutoBookPlacer.esp6E  Curseman's Anti Extinction Mod.esp6F  Dungeon Actors Have Torches 1.6 DT.esp70  seorin's Artifact Fix.esp++  sixier Sheogorath.esp71  Enhanced Vegetation [100%].esp72  Enhanced Economy - House prices.esp  [Version 1.0]73  RealSleepExtended.esp  [Version 2.4.1]74  nGCD.esp75  nGCD Oghma Infinium.esp76  nGCD Skeleton Key.esp77  1em_Vilja.esp78  bgBalancingEVCore.esp  [Version 10.52EV-D]79  bgMagicEV.esp  [Version 1.7EV]++  bgMagicSpellTomes_for_WryeBash.esp  [Version 1.68EV]++  bgMagicItemSigil.esp  [Version 1.68EV]++  bgMagicEVAddEnVar.esp  [Version 1.68EV]7A  bgMagicEVPaperChase.esp  [Version 1.68EV]7B  bgMagicAlchemy.esp  [Version 1.57]**  bgBalancingEVOptionalNPCDiversity, TNR Merge.esp  [Version 10.0EV-D]**  bgBalancingOptionalNPCDiversityKvatchRebuilt, for Wrye Bash.esp  [Version 9.0EV-D]7C  bgBalancingEVLAMEAddition.esp  [Version 10.51EV-D]++  bgBalancingEVLAMEGlue.esp  [Version 10.0EV-D1.62EV]**  bgBalancingEVOptionalNPCDiversityLAME.esp  [Version 10.5EV-D]++  bgBalancingEVOptionalSeamReducerHighElfFix.esp  [Version 10.0EV-D]7D  bgIntegrationEV.esp  [Version 0.993]7E  TheOubliette.esp7F  bgMagicEVShader.esp  [Version 1.7EV]++  bgMagicShaderLifeDetect.esp  [Version 1.68]80  bgMagicLightningbolt.esp++  KoldornsImprovedLava2.esp81  Enhanced Economy - Soulgems.esp  [Version 4.2]82  Real Hunger, Cobl.esp  [Version 1.6.1]83  StokerWolff.esp++  Cobl Filter Late MERGE ONLY.esp  [Version 1.53]84  Bashed Patch, 0.esp85  Oblivion Graphics Extender Support.esp  [Version 0.2]86  BetterCoins.esp87  DLCFrostcrag_MysticEmporiumPatch.esp88  FormID Finder4.esp
 I'm hanging onto a save from just before my character accepted "The Hist," so if there's any more testing that needs to be done, let me know. If I get really ambitious I may start waving Wingrep at things and see if I can find any other mods that might have their sticky paws all over Maglir. Probably won't happen tonight, though. My eyes are starting to cross. 
