Turning a scroll into runestones positively identifies the scroll in the game so that you’ll recognize it automatically the next time you see it. This is because each scroll turns into the same runestones every time and runestones are always identified.

The same is not true for potions. You have to actually drink the exotic potion to identify it. Making it exotic just makes that task safer (and avoids wasting a precious cleansing potion).

You can however identify potions via alchemy another way: turn 3 seeds of the same type into a potion will identify that potion for you. This can be an absolute game changer if you find 3 seeds of mageroyal early on but that’s rare. Getting any potion identified for free is valuable though, and using seeds to do it is totally free (doesn’t cost any energy) plus with the Barren Land challenge you can’t plant seeds anyway.

Seeds do have other uses besides making potions. Using them to tip darts is another extremely useful thing to do in challenges (and can be totally ignored otherwise). Seeds of blindweed and earthroot are especially valuable for tipping darts because they give you blinding and paralyzing darts respectively. Those two darts can save you from a lot of damage at the hands of annoying ranged attackers like gnoll shamans and dwarf warlocks.

I actually don’t use aqua blast against Tengu. There’s another potion you can make which is extremely valuable for that fight (and 2 other boss fights): shrouding fog. This is the exotic version of invisibility and it’s extremely good at protecting you from ranged attacks while still letting you fight normally (unlike invisibility which goes away the instant you attack someone).

Aqua blast pays for itself even if you use it to loot just one of those poison gas treasure rooms. Its other uses for putting out fires, attacking flaming enemies, and creating water (which you can use with aquatic rejuvenation and/or glyph of flow) are more specialized uses but super strong if used correctly. If you have a wand of lightning then aqua blast becomes extremely strong as a combo to get enemies into water so you can fry them all with lightning arcs.

Returning to shops is the only way to buy stuff from them. Going forward to a future shop will not get you the items in the previous shop. When you play with a lot of challenges you tend to want to get as much value as you possibly can out of each shop. Plus the prices at the shops increase the deeper you go into the dungeon. Buying anything from the first shop (in the prisons) is far far cheaper than it is from the ambitious imp (last shop in the game). A potion of healing costs 300 in the first shop but 750 in the last shop (goes up by 150 each chapter).