My suggest are sell every color soul you get except of yellow soul, then use 30000 soul to level up your character, and rest of it to Leaf, your companion level up much faster and can pretty much carry you all the way.
Using the Golden and Silver Rings of Avarice, farm the Pied Piper's Plague Rats using Soul Radiance to AoE one-shot them. 1600 souls per fight, 4 chances of a Yellow Soul dropping. Farm them for awhile and you get tons of Yellow Souls to use on yourself or sell once you get enough speed, and tons of souls to buy whatever you want, or massively level up, or whatnot. Did this and got enough souls to level up Leaf to 7X and myself to 5X, easy. There's a bonfire right nearby that you can use to refresh, too, and if you want to hit up the four mobs above that area the rats drop yellow souls and bleed-stoppers, the slimes drop blue, and the man-eating sharks drop red.
Hanzel and Gretel case is probably the worst offender for locking out. You can miss some things, but there's almost nothing significant like that. Using colored souls has no negative consequences.
If Crestfallen Hero left the Holy Forest, you may find him right past the mayor's house in the Rotten Burg.
It's really not that bad. Unlike with Dark Souls, you can save anywhere, at any time, and they're right at the start. The bonfire is also right there, and if you pick up any of the breadcrumbs, Leaf, right beside the bonfire as usual, will tell you that you fucked it up.
EDIT: Huh, pretty sure no one else has mentioned this before, but in Snow White's castle, you can follow a White Rabbit in the upper left to Wonderland, and in the upper left THERE, there's a door in a waterfall, where you can find a corpse against a tree (this is Jabberwock, if you attack it); Talking to it after "killing the string-pulling fiend" (unsure who this is in reference to, but I haven't killed any of the Princesses or superbosses) lets you give each of your companions their Ultimate Skill for 10,000 Souls each.
EDIT 2: The Nameless Trader in Gothel Tower second floor upper right corner is Elma's father. If you spare his life, and possibly activate Elma's flight to Holy Forest, he shows up there below and a bit to the right of Goose's stall. He sells Divine Blessings, Magistones, Magical Mirrors, Fire/Ice/Lightning Pine Resins, Fairy's Wings, and Succubus Fluids. Notably, he is also willing to buy your items, so if you kill the Prayer Master for the covenant, you're not screwed for having someplace to sell items for souls.
EDIT 3:
Can't beat Rapunzel either. My pc (player character) and all summons are level 50-55 with ~3000-4000 HP. I already beat 3 princesses, but that one is dumb. I can get to phase 2 pretty easily as well by throwing a kinds of damage buffs on the pc and then spam "Omniblow" (seems to be the only attack that can hit her). However once she enrages, she just knocks down one party member after another. Especially when she starts healing and takes 3 extra turns in a row every time, the fight is pretty much lost.
Already got a weapon at +10 too with +50 Attack Boost ring.
You can get Dispel from a magic seller, IIRC Dorothy after you recruit her. Might be the Shaman, though, can't remember for sure. Makes Rapunzel a joke. Also a good idea to covenant her first (since her covenant is simple and doesn't require a sacrifice) as her ring gives something like a 90% evade chance and double actions, if you haven't already powerlevelled yourself to be able to defeat her through sheer force.