Wiki is rather poor in content so I have to ask here:

Apparently there is a ex-slave female companion in Secret Cave but I have not the slightest idea where it is, by this point I've done most of the quests and explored almost entirety of map and have not met her. Any help?

Another thing I have to ask: In the first elf village we unlock(right below our capital) there is a girl talking about a secret path in Calbridge Stronghold, there is also a guard that mentions it, but I haven't been able to find it, does it exist or is it just fluff that hasn't been added yet?

After getting Manticore in barracks I thought I will unlock special monster girl squadron, but it doesn't seem to be the case. Are there any squads that can be added? Or is it gonna be added in futute updates? 

I just don't know if I am missing things or are they not implemented yet so sorry for this flood of text

Edit: The game is asolutely wonderful! Hope we get to see some more kingdom managment soon! Also you may want to nerf gold gain from Arena, 50 000g for every turn passed as long as u win some easy combats is absolutely broken.

It's not a secret cave. If you had actually explored most of the map you would have found her. Maybe you can find a walkthrough somewhere. If you want to know about the secret hot spring you can look at past posts here where it is explained in full. (Or in a walkthrough if you manage to find one)

You can download the official walkthrough for free. It tells you how to get all companions and how to unlock secret path in Calbridge stronghold.

As for the manticore, so far there's no more content, at least not yet.