Alright, let's go through these one by one.
1. The "fence north of the longest column" can be found south of Oasis, in the south-west part of the desert. There's a screen with four columns of plants, the longest of which is four plants in a row. The treasure can be found next to the fence, straight up from those four plants.
The "Dead end, right of stairs" is at a maze in the south-east part of the desert. The maze is spread across four screens, and the treasure is in the top-right screen, the one you enter the maze in. It's one tile to the right and one tile down from stairs.
2. In the Oasis Church, on the second floor, there's a room labeled "Tivelian Altars". One of these altars has a basket in front of some bottles, where you can place various drinks. Select "All At Once" when you have a Bristleback Beer, Tivelian Ale, Oasis Special, Gut Buster and Fine Wine in your inventory. It'll take all those drinks and give you Nectar of the Gods in return.
3. First you have to pay Rich Goldman before he'll even speak to you. Then he won't continue unless you have enough of certain expensive items in your inventory or equipped. Any item or equipment that has "gold" in its name counts, and others that also count are: Diamond Ring, Stylish Cape, Top Hat, Bottle of Korenta '54, Stone of Brisk Breezes, and Venom Apple Juice. Each item should show unique dialogue if you have it, including Dirt Figurine which I didn't mention because he actually counts it against you as a gag. You don't need all of these items, and can be missing several and still get Rich's approval. The reward is a unique shop.
4. The paper in that room has a note that says "spell lock: 2:00, 8:00, 4:00, 10:00", which is a hint that you can interact with objects in the room with those positions on a clock. Check the shelf in the top-right, then bottom-left, then bottom-right, then top-left. When you do a noise will play and the pot will open. Check it for some items.