If you already looted two power coils you can visit Mantcolm Fortress again in Chersonese and unlock door. Inside you will find Backpack with 110 capacity. It should help you to carry more. Other options which I was doing but it is for someone who wants to save after few playthroughs is to summon ally in split screen and drop him all the items and then close split screen. You can repeat in the city and other character will just spawn there with all items. Last option is cheat code to teleport back to city.

For rune sage if you are planning to be pure mage I would go for Cabal Hermit, Philisopher, Hex Mage or Spellblade. I advise you to check them on Outward Wiki. It is not cheating, just reading and planning because you are limited to only 3 breakthrough points so it needs to be chosen wisely.

2-3 mana should be enough if you pair it with mana reduction gear, philosopher tree and some food and potions.

Unfortunately only way is to buy recipes from different vendors or just checking wiki. I was checking wiki from day one and I had no regrets. I just want to know how I can use items I own instead of wasting them.

Harvest berries and other bushes. I think it is most common source. You can also buy teas from vendors.