Honestly, I didn't realize any of those options to spend money existed--I saw the other doors in town, tried to open one and it didn't open after pressing buttons (and it wasn't obvious it was interactable in the first place), so I assumed the buildings other than the main one were just decoration. I did find the one conversation with Frankie, but other than that it didn't seem like I could do anything in town other than talk to people.
Oh, so everyone was max level in the version I played? That would explain why I didn't have any problems completing it without spending money (my characters didn't seem to take much damage in the prep phase, and I started with more than enough characters to cover all six slots at once). Level 5 being max level also didn't seem blatantly obvious, between DSD being L99 (obviously none of my characters would get there, but a little advancement wasn't implausible) and each of the bonuses in the boss battle only being ~one bar.
The bottle seemed like it was doing something, but it wasn't obvious what made it better (maybe it was ranged?). The spatula didn't look like it was doing anything--I'm assuming it was a slow attack compared to the fast attack of the cleavers, but it wasn't practical to use since you're dodging all the attacks (and I don't like slow melee attacks in general). I think what made the cleavers the best is that they had an actual attack animation that was responsive--the bottle didn't really.