I really like Mystical Cunning over PES myself but PES is always a good choice. Flex is definitely #1 because of the interaction with Expose Weakness, even after that gets fixed it'll still be good.
Regarding Spine of the World, only Gorbat, Argoniel, EoR endboss and the water dragon in Flooded Cave have it. So at least you can murder Elandar with your poisons etc.
As for individual categories:
Dual Techs - Obviously get Flurry at least. Heartseeker can act as a pseudo rush which is nice for quality of life purposes as it can pop trash mobs from stealth instantly, and Whirlwind is one of your very few AoEs as well as a sorta escape. Not 100% required but I like to get at least 1 each here.
Duelist - Parrying attacks prevents them from critting so level this asap. It's random but massively reduces damage taken overall. Plus some free offhand damage. Tempo is decent for a little more stamina but doesn't need heavy investment. Feint and Lunge are actually pretty good at defending vs melee enemies, disarming powerful melee mobs really disables them.
Throwing Knives - Niche, but fun for a gimmick build. 1 point is nice just to plink away at things at low levels though if you haven't got a good conjure wand. Note that heavy investment to get Quickdraw is actually a substantial damage boost, but it's a lot of points.
Poisons - Vital. At a bare minimum get enough for Numbing+Crippling as it's a massive boost to defense vs things that can't resist. This is where Mystical Cunning can help, although areas like Daikara and Dreadfell are a pain. Leeching+Venomous Strike is actually decent healing.
Trapping - Like bpat said, 5 in the first talent is pretty good as there's some really nice traps. It's tempting to go for the final talent though, as that effectively gives you a Poison Grenade that can layer on all sorts of status effects. I'll probably buff Decoy at some point so you can 'load' a trap into it that detonates when it's destroyed.
Lethality - Expose Weakness is currently brokenly good so use it while you can. Basically any additional melee hits like flex combat etc stack the buff so you can get an absurd amount of +damage. Blade Flurry is a solid boost if you can manage the drain, and Snap is very good with your big talents like Shadow Dance, Rogue's Brew, Marked for Death etc.
Stealth - Unreliable but good with light investment. Shadowstrike gives a large amount of crit mult, Soothing helps with stamina, and Shadow Dance is a strong buff. Note that vs unseen attackers you have a 2/3 chance to miss with melee attacks, so chaining Shadow Dance via Snap can help vs melee mobs.
Artifice - Probably the first thing I'd unlock. Rogue's Brew is vital as it gives healing, stamina, cures, a mini-heroism via the mastery talent, and can be reset via Snap. Slots 2/3 are flexible, I like Hidden Blades/Grappling Hook but it's worth experimenting. Note that as HB trigger unarmed strikes, finding a pair of gloves with strong on-hits like Dakhtun's, Scorpion or Cripple/Dominate randarts is worthwhile.
Assassination - Optional but I like unlocking it. Coup de Grace deals percentile damage - 40% / rank of missing life at 4/5. So imagine a boss with 5/10k life, that's +400 damage multiplied by your +physical etc. Can really add up, not to mention the instakill which is nice vs necromancers etc. Terrorize is nice but unreliable, Garrote is a good amount of additional damage and can help vs casters. Marked for Death is weak at low points but super strong at max. Can easily see 5-10k damage in 1 shot from it.
Dirty Fighting - Niche but doesn't require a lot of investment. Would recommend just grabbing Backstab for +30% damage or so, and Twist the Knife to deal with buffs. Good vs stuff like Vitality.
Mobility - Max (well, at least 4/5) Trained Reactions asap. Tumble is also great.
Scoundrel - Decent if you want to unlock it, probably only would on a Cornac and even then tinkers are probably better. If you do, 1/2/1/2 minimum. Lacerating Strikes is ok but nothing amazing. Scoundrel's Strategies is a ton of crit reduction although the talent CD doesn't do loads unless you're critting every turn and they don't have tons of talents. Misdirection is fun for reflecting stuns etc although unreliable. Fumble doesn't gain much out of going past 2/5.