I think choosing 1 of the boys and their pet is only available after finishing all the Goals. It doesn't have any other requirements, so just getting it without doing the work wouldn't make sense.
And about the Mithril.. it's the same as a lot of things in the game. They are rare when you need them and not so later on.
For example, in the beginning Golden Items are super rare and really hold up your progress. Now I use them to craft Gems.
Fruit is rare, and you think you have to buy fruit trees.. but they drop from Adventures pretty often.
Then Spellbound Dew is a rare item, until Main Story Line rewards + event prizes + MRC animals gift you with guaranteed Spellbound Dew drops.
I remember needing the Orange Coral/ or shell? for the mermaid storyline.. after finishing the storyline you're rewarded with stuff that drops the coral/ shell, but you won't need it as much anymore and will max on it soon..
The same is true for Mithril. I'm maxed on it, and if I spent it I've got a couple of guaranteed Mithril droppers. 1 is a reward for completing one of the Main Story Lines, some are prizes in events and some are from Baron's Tent prizes.
Eventually you will get enough (of whatever is holding you up, Mithril in your case), the game is designed to have several of these 'rare item hold ups'.
One of the satisfying things about this game is acquiring things that drops what is rare for you at that time.
Keep working through Main Story Lines (they have excellent rewards!) and gather prizes wherever you can. (Cabbage Hut is a very good investment imo)
I think leveling your Sparkling and Deep Mine to Level 9 will also help. Good luck!
And try to enjoy the challenge. I miss it sometimes.