SP regeneration fixed in 1.5 (or maybe earlier). Shops scale with difficulty and complexity in the meantime.
Charisma should increase the SP regeneration, but it doesn’t seem to. I tried adding dozens of points to charisma, but didn’t see a noticeable difference in how fast the points regenerated.
Also, there is no way to make prices in shops scale with charisma as the conversation script can’t read the player’s charisma value (or I didn’t find a way to do it).