Mystic Weapon Finesse? Do you mean Mythic? If so be warned that that's only available to mythic characters. If not, what is that from?
What level are you starting at? You're going to be pretty poor at stabbing people until ~7th level as written. If stabbing people early on is important you might want to rethink.
Extend/eternal potion affects potions created via brew potion, not alchemical items created via Craft: alchemy or extracts.
Mythic weapon finesse (& a +dex mutagen) will help with stabbing then. You're not likely to have spare extracts to give your friends at 3rd level anyway. I was going to recommend slashing or fencing grace assuming the mythic feat was an error, but you won't be needing them.
Fencing grace applies to a rapier, slashing grace to any one-handed slashing weapon, and the latter makes class features/feats applying only to 1H piercing weapons (you don't have any) apply to your slashing weapon also. If you need to get either you might consider a 1 level dip in the inspired blade archetype of the swashbuckler to get the prereq feats fast, and to get the opportune parry & riposte deed.
BTW, mythic not mystic!
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1 Inspired blade swashbuckler/ X Emp. investigator
First level Fencing Grace.
Focus on dex and take mutagen and quick study @ 5.
Mutagen + cats grace + reduce person= +10 dex, +2 NA, +1 size bonus to hit and to AC. At level 5...it is a huge combat boost.
Basically you will be boss hog with insane amount of skills and an insane dex to do damage with. Let's not forget parry and ripose option for combat and to mix of inspiration and daring do for skill boosting.
You can also work in potion glutton by freeing up so many feats.