1.) Rogues do not get immunity to flanking at level 4. Uncanny Dodge at level 2 makes them retain their dex bonus until they act, so they are never considered flat footed even against invisible enemies. Also you can still lose your dodge bonus when you get feinted successfully. At level 8 rogues get Imp. Uncanny Dodge which gives a conditional immunity to flanking if your rogue level is not 4 levels lower than the opponents. Your TSS/Thug example would get that level 9, and the build I was talking about before you chimed in would have gotten it at 13. I checked the list of rogue talents and cannot find any talent there which gives immunity against flanking at level 4. This of course means that splashing in other classes severely reduces the reliability of Imp. Uncanny Dodge.
Uncanny Dodge at level 2 makes them immune to being flat footed, so they only suffer sneak attacks from being flanked which you can prevent. Being flat footed is one of the biggest sources of sneak attack damage for the enemies, since it happens at the beginning of every combat if your initiative is less than an enemy. Immunity to flanking comes in at level 8, or 9 in the case of TSS1/Thug8, which is considerably earlier than TSS13.
2.) There is no advantage one way or the other when it comes to feat. When you are swimming in that many feats you can get whatever you want. Rogue gets a few extra special feats but fighter gets a bonus feat already at level 1.
No, Rogues definitely have a bigger advantage here. They can select any non-fighter specific feat, their own small selection of bonus feats, and at 10th level they can also select improved rogue talents.
3.) Multiple bonus for movement stacks up. More importantly Haste also cannot give you more than double your speed, so the difference is 20 feet further travel distance, 40 vs 60, or in other words 50% more speed just from that and it goes on from there. Yes it does matter, the faster you get into melee the faster you get your AC bonus from defensive fighting. Movement speed also limits your charge range which is very useful when facing scattered groups of enemies.
Haste gives you +30 Enhancement. Other sources of Enhancement do not stack, only the highest applies. This includes Swiftwind and Onslaught which are also Enhancement. 50 Movement is sufficient to charge across 80% of the entire screen width wise, and exceeds the screen height wise (4K). Manticore Boots are +10 [Other], so achieving 60 on your tank is common and easy beyond the early game. 60 Movement can cross an entire 4K screen with a single charge.
4.) A proper tank rarely if ever gets critically hit. Having guaranteed 5 points subtracted is a noticeable reduction, even if the enemy would hit for 50 it would still be a 10% and few if any enemies hit normally for 50 damage from a single damage type, at least physically and if elemental damage is involved that is even easier to reduce.
50+ is quite normal on Hard or higher difficulty, with crits exceeding 200+ (damn Undead Cyclops). You might have an argument if you were talking about the early game, but you don't get 5/- until Level 19. That's nothing. You literally get a helmet with 10/Cold Iron muuuuch earlier.
5.) Human fighter has up to 22! feats to chose from. You can easily fit in Skill Focus: Persuasion if you feel it is needed. Not really an advantage for thug either.
Valerie with TSS1/Thug19 also gets 22 feats, and offloading Skill Focus: Persuasion onto your Rogue side is better on her since she starts with Dodge and Toughness already consumed. You can also give her Skill Focus: Trickery without negatively impacting her basic feats if you feel so inclined.
6.) Base reflex save for rogue is +12 and it is essentially +11 for TSS. That means that in addition of having near rogue like base reflex saves you also have a strong con save, leaving only will save as your weak spot which is the same for rogue/thug. How is that not vastly superior?
Valerie already has massive con, which keeps growing with Physical Perfection belts. She almost never fails a fortitude save. Having an even higher fortitude save is unnecessary on her.
No TSS1/Thug 19 is not considerably superior. It deals superior damage when flanking and adds some utility I can get from at least 4 other character, Linzi, Jubilost, Nok Nok and Octavia, that's about it. Worse HP, worse AC, Worse con save, no up to +8 bonus AC against touch attacks, far worse BAB since you lost +5 BAB and still have the tower shield penalty and do not get the weapon specialization feat series which adds to another +1 to hit and +4 to damage and of course one attack less at a much higher BAB, namely +20 instead if your +15. So just from that you now lost +8 to hit baseline and an attack in total. Congrats, want a cookie?
You're flanking almost all the time with summons, so it's dealing superior damage almost all the time. It adds dramatically more skill points and - spoiler alert - Linzi dies. Worse HP is still higher than everyone else in the party, including Level 20 Amiri. Having lower hit DOES hurt, but with Fox Amulet and/or Brilliant Energy you hit on everything but a natural 1 against Wild Hunt (which is the dominant enemy for two entire chapters). The only thing you have any difficulty with at all is undead since constructs are incredibly rare.
So yes. I want all your cookies.
Even though they are probably oatmeal raisin.