Multi-limbed creatures and mixing Multiweapon Fighting with Multiattack

[kevin video]

So I've got a question that will probably be easily answered by you guys and your thankfully infinite hive mind of knowledge.

A marilith's bonus with a six longswords is +25 across the board. Her six slams are +24. What happens when she gets disarmed of one or two of her swords, and she has to use slams in place of the lost weapons. What are her bonuses then? She has both Multiattack and Multiweapon Fighting. Does this mean that no matter what it's just the same -2 regardless? Or, now that she has one, two, or more secondary attacks, do those become -5? What about in conjunction with her tail slap? Does having both feats help with this? Would having just Multiweapon Fighting have sufficed?
What about a thri-kreen? How does that work for its claws and using individual weapons at the same time? Would you need both feats then too?

[kitep]

I think the tail slap answers the question.  She gets +25 with the swords, and +22 with the tail slap.  If she uses a slam instead of one sword, it would be at -5 normally, but reduced to -2 because of multiattack, so it would be at +22 with a slam, and half str damage as a secondary weapon, so 1d4+4

I don't think the multiweapon Fighting helps with the slam when mixing with swords (though it still helps with the swords)

[kevin video]

Okay, glad I'm not insane. I'm looking at a 3PP build of a monster they gave daggers to, and the damage is twice as much as a creature of that level should deal. Granted it's Pathfinder, but the rules still apply.

[kevin video]

Let me share the build. Again, it's Pathfinder, but it's an advanced HD with the advanced template leukodaemon that had four arms added to it. It's supposed to be a "deity" so it's also got +5 to its regular DR and element resistances, as well as it has the rend ability. I know it's wrong. I'm just not 100% exactly sure what's all bad on it. I've figured about about nine of its problems. Like, missing two feats. Breath of Flies is all weird too compared to the original. I have no idea where he got the numbers for that. Also, in Pathfinder, a CR 15 should only do 70 damage on average per round. This does over 100 if it gets a rend off.
For those who've ran the Way of the Wicked AP, playing it, or plan to do either, I'm hiding it under spoilers.
Here's the original leukodaemon to compare this "upgraded" one to.

(click to show/hide)

Vetra-Kali Eats-the-Eyes, Daemon Prince of Pestilence CR 15
XP 51,200
NE Large outsider (daemon, evil, extraplanar)
Advanced unique six-armed leukodaemon
Init +13; Senses darkvision 60 ft., deathwatch, detect good; Perception +24
Aura infectious aura (50 ft.)
DEFENSE
AC 27, touch 18, flat-footed 18 (+9 Dex, +9 natural, -1 size)
hp 189 (14d10+112)
Fort +13, Ref +18, Will +16
DR 15/good or silver; Immune acid, death effects, disease, poison; Resist cold 15, electricity 15, fire 15; SR 24
OFFENSE
Speed 30 ft., fly 60 ft. (average)
Melee bite +18 (1d8+9), 3 claws +18 (1d6+9 plus rend), 3 large +2 corrosive daggers +21 (1d6+11 plus 1d6 acid and contagion)
Ranged 3 large +2 corrosive daggers +21 (1d6+11 plus 1d6 acid and contagion)
Space 10 ft.; Reach 10 ft.
Special Attack breath of flies, rend (3 claws, 1d6+13)
Spell-Like Abilities (CL 14th; concentration +19)
Constant – deathwatch, detect good
At will – contagion (DC 19), dispel magic, greater teleport (self plus 50 lbs. of objects only)
1/day – harm (DC 21), summon (1d3 leukodaemon only, 100%)
STATISTICS
Str 29, Dex 28, Con 27, Int 20, Wis 25, Cha 20
Base Atk +14; CMB +23; CMD 42
Feats Alertness, Hover, Improved Initiative, Point-Blank Shot, Weapon Focus (dagger)
Skills Fly +24, Heal +24, Knowledge (arcane) +22, Knowledge (planes) +22, Knowledge (religion) +22, Perception +28, Sense Motive +28, Stealth +22, Survival +21, Use Magic Device +22
Languages Abyssal, Common, Draconic, Infernal;telepathy 100 ft.
Gear 3 +2 large corrosive daggers, tears of achlys
SPECIAL ABILITIES
Breath of Flies (Su) Once per minute as standard action, Vetra-Kali can unleash a cloud of corpse-bloated, biting black flies in a 20-foot cone. Those caught in the cone take 10d6 points of slashing damage. A DC 23 Reflex save halves this damage. Those who take any damage are also sickened for one minute. In addition, the flies linger for 1d4+1 rounds, congealing into a buzzing 20-foot square cloud centered on the cone’s original point of origin. Any creature that ends its turn in this cloud must make a DC 23 Reflex save to avoid taking 5d8 points of damage and becoming sickened for 1 minute. This cloud of flies may be dispersed by any area effect that does damage or creates wind of at least strong wind force. All daemons are immune to this effect.
Contagion (Su) Any dagger Vetra-Kali wields is tainted with disease. If a creature is damaged by a dagger it must make a DC 21 Fortitude save or be affected as if by the spell contagion. Any creature need make only one save a round regardless of how many dagger hit it.
Infectious Aura (Su) All creatures within 50 feet of Vetra-Kali take a -4 penalty on Fortitude saves against disease effects.