Settle on some idea - any idea - and I'll lend a hand to try to make it good Julius. If this is just idea #11 of 20 I'll wait until you get to #20.
Hmm. I really can't believe you've used half the 15th level builds you've asked about recently. If this one's different, fine.
Mithral kukris not silver BTW. Mithral is silver for DR & so for silver vulnerability, but they don't suffer the penalties (-1 damage, lower hardness/HP) of being alchemical silver. The extra cost is nothing at 15th.
You might want the mithral current slice feat and so its prereqs.
With one good save (Fort) and minimal save benefits from your maneuvers you may want to spend a feat on iron will perhaps, or even another on improved iron will. Or if you're planning to fight unarmed and not to use two-weapon fighting you could use one hand carrying a padma blossom.
Looks OK given your starting level. Shake it off is a teamwork feat which boosts saves. Forget the padma blossom, I had a momentary brain fade about what style of combat you were looking at.
That dip saves a feat (net) and gives you +2d6 sneak attack & evasion but it also costs you your 8th level maneuvers, a maneuver known & one readied, and makes your ability to grant that teamwork feat still a standard action rather than letting it become a move action. If you play to L20 you'll get most of those back but you'll lose some other things.
Looking at it right now the biggest loss is not getting your 8th level stance and an 8th level maneuver. mithral lightning stance and adamantine fang look like significant losses to me - the latter is a +4 to save DC with any poison or prana boost used with it.
The way the warlord's maneuvers known works, including their ability to swap a maneuver known at each even numbered level, is that at class level 15 you have one 8th level maneuver and 2 of each lower level until you run out of the 14 total maneuvers known. 1 8th, 2 each 7th, 6th, 5th, 4th, 3rd & 2nd, and 1 1st. For stances you have 2 1st level, 1 3rd level, 1 5th level, 1 6th level and 1 8th level.
Assuming you get the 8th level ones I named above and stick to your 3 named disciplines, and you want poisons in particular my thoughts are:
7th: Numbing venom prana is an OK poison. Beat the clock is too good to miss.
6th: Endless current works well with your TWF, as does blend with the night. For the stance, sand bearer's swiftness gives you haste when you need that. Sting of the viper does charisma damage which has little synergy with anything you're getting BTW, I wouldn't recommend it.
5th: Burning venom prana works if anyone else in the party targets will saves. Flowing stream is one of the very few save boost/replacers you can get. Chronal fission is a stance which may be useful sometimes.
4th: Temporal adjustment is a useful counter, sting of the adder synergises with burning venom prana, or ride the current stops full attacks against you. Pick two of these.
3rd: Night's knife is a useful boost and probability twist is a useful counter. Steel coils is a stance which helps you grapple if you ever want to.
2nd: Iron wave lets you make an attack at range, fading dodge saves you from a hit while also letting you debuff your enemy.
1st: Flowing creek is another save yourself from a hit ability which you may ready once in a while even at this level, and you likely want both steel serpent stances - a bonus to stealth is something different, and a huge bonus to poison save DCs is the best bonus to poison use of any of the stances.
Remember that with poisoner's stance and adamantine fang you can get +11 to the save DC of a poison. Even drow poison (DC 13 Fort) may be worth using, and it's both cheap and effective if the target fails their save. Leng's tears is a more expensive, higher save DC poison. Deathblade is even higher DC though the effect is less.
Purple worm poison generally isn't for sale, there's no price listed. Presumably no one has a good way of farming vast burrowing creatures and extracting their poison.
You already have a thread asking about how to spend that 300K. The difference here (besides getting poisons) is that you need to buy 2 weapons, which is expensive - maybe get one good kukri, +5 keen and one cheaper, +1 ghost touch injecting?
If you get something like that list of maneuvers then you'll need to make a perform (dance) check sometimes. You might like a trait which helps with that. Also remember to max ranks in both perform (dance) and in heal; you may never make a heal check but the number of ranks in it matters for some steel serpent stuff.
I have no particular opinion on gambits, tactical presences or other warlord class features, pick whatever seems appropriate to you.