few questions...

[Elwin]

1, Yes you missed quest. IN hut next to castle there is fredericks wife kept, if you free her and bring back to frederick she gives a quest to atakc castle

2. Missed again .. its a main quest part, he got one of stones

4. Merchant in tekron on the coast, its impossible to miss it

[DGDobrev]

3. Yes, it is possible. On the second attempt (you always lose the first one), when there are between 1000 and 5000 gold on the table, summon the thing your opponent used to beat you the last time and you win.

[TemjinGold]

4. That potion (Potion of the Goddess) can also spawn in the shop near Zebat's castle. The Tekron spawn is guaranteed. I got both in my game (since I didn't plan on using Jimmy, that's 300 leadership for me!)

[Elwin]

Second one can spawn anywhere or no at all.