The third example sentence for たら if/after/when is
もし食べに行きたかっ たら 、行こう。
which is translated as
If you want to go out to eat, let’s go!
I don’t understand what we’re conjugating? My guess is the “~たかっ たら” part is corresponds to the “want”, but the only “want” I’m familiar with so far (21 lessons into N4) is ほしい.
Help or pointers appreciated!
Hey! 
Great guess! Understanding from context means that your Japanese ability improves 
There are other “want” constructions and one of them is used in this sentence - たい 
It is used with verbs(I want to do something). While ほしい is used with nouns(I want something).
https://bunpro.jp/grammar_points/59
The たら form of たい is its past form たかった + ら - たかったら。
There is 3rd “want” grammar point “てほしい” used when we want someone to do something.(I want person X to do something)
https://bunpro.jp/grammar_points/101
Oh, duh, I had already learned that! I guess I’d just been drilling on ほしい so much recently (including ~てほしい) that I lost sight of ~たい.
Thanks so much! It’s great knowing that when I get stuck on anything in my Bunpro-driven grammar studies, I won’t just be lost, because these forums have my back.
We are doing our best 
I’m in doubt here about the conjugation.
行きたい means to “want to go”
行きたかったら means that “if you want to go…”
How do I say “If you wanted to go”? Like in “If you wanted to go, you should have said so”.
行きたかったら言ってくれればよかった（のに ）。
のに as but/although. It will add a meaning of:
“but you didn’t tell me”.
Full version(since the part after のに is omitted in cases like that).
行きたかったら言ってくれればよかったのに言ってくれなかった。
Thanks for your help!!