I'll respond to A-G like open ended questions.
A: Good energy shields are good in every build. Optimized builds will swap between a Low and High speed shield in endgame, I prefer Low while in early-mid game. Electronics also is critical for psi headbands. For the headband options: your most spammed damage school, your primary damage skill, and psionic mufflers given Tranquility can be limited by psi points. 45 Biology is all you need for infinite adrenaline and focus stims. Get more if you want.
B: 50 pickpocketing is usually enough to get ammo/meds/oddities off random guards in cities. It all depends on Perception, some need way more, tweak points upwards to taste. 100 base lockpicking and hacking are usually enough, some crazy endgame containers exist, going lower has merits, going above is for the truly obsessed outside very particular checks. You can stealth past most enemies with crafted gear, but more makes it easy.
C: Honestly, I'm not a psi expert. Tranquility is good, Neural Overload is insane. You can check more here
https://docs.google.com/document/d/1Twqp6lMtdmvJrLyAgJ1UZoBbNgO4HJ_0mXQY-zrpro8/ . There are some really powerful psi staffs in Black Sea. You can potentially learn multiple psi schools and swap which are used, but that's overkill. Every school combo can potentially work, and every school can go solo.
D: The biggest gripe most people can have is how its hard to make multiple weapons work better than optimizing one weapon. Specialization is hyper encouraged by Underrail's systems. As said above there are even staffs that exist to eat up weapon slots. The upcoming DLC might make 25mm daze grenades from a grenade launcher a low investment crowd control tool. To get all the feats required for both a gun, and psi build you'll need to cut goals like 16-18 Will. Shotguns, and Sledgehammers have the lowest ability requirements to function as a side note, but its unfortunately a "swimming upstream" moment, and is more of a choice for flavor than power.
E: The game throws unavoidable combat encounters to "check" your build no matter your choices or social skills. Most social skill's power are judged by how they relate to combat. Intimidate keys into Yell, Mercantile is mandatory to get the best crafting materials from vendors (105-110), but Persuasion really comes down to a few neat but not mandatory options for some quests.
F: The best trap is Bear Trap and doesn't need any skill. However if you go big into traps, then Biology gets more use for poisoned traps.
G: Throwing nades, and nets are fantastic on every build though, 123 skill to have max accuracy at max throw range.
A strong build isn't necessary to win. The game has many universal abusable systems: traps, utilities, doorways, corners. A strong build lets you avoid abusing these things, and instead can coast off its built in power.
