1) I think you lose to her if you want to advance the slime girl quest line. Honestly you can just try both options. You can't permanently kill the slime girl so even if you defeat her, you can just come back later and lose or surrender to her on the next attempt.
2) Just keep buying stuff from Lucas. His sister will eventually show up.
3) I don't think there's any more info from the stone. At this point i think it's just used for the cat. (I could be wrong though, since I haven't finished the latest update yet.)
4) Don't think so yet. (see previous answer though)
5) Pretty sure you can't do anything with magical beans yet. Electric shard is used much later in the game (after you find elf village) to acquire an AoE spell.