I need to somehow make hunter mend pet on cooldown (for soloing some bosses). Also maybe the 30% heal spell + turtle. Is there any easy way to add these without having to write a whole script?

No, if you want to add to a priority, you have to have some comfort/familiarity with APL syntax and logic.

That said:

1. Go to /hekili > Priorities > Beast Mastery (or whichever) > Action Lists.
2. Select Action Lists tab.
3. Select the default Action List.
4. Click the + next to Entry to create a new entry.
5. Select Mend Pet.
6. For conditions, probably try pet.alive & pet.health_pct < 30 and tweak as you see fit. That literally means, "pet is alive and pet health is below 30%."
7. Go test on a boss dummy.

Note that when you edit a default priority, it will be overwritten if I release an updated priority later on. To get around that, you can create a copy.

1. Click
   

on the priority's screen, next to the name:

2. Click Accept.
3. Change the name to something clearer than "Beast Mastery (2)" if you want.
4. Check Active. (This will tell the addon to use your edited copy.)

Thanks!