Zaldia only give a diamond for pass purchases with real money, or the every-thirty-days-pass too?

Originally, as far as everyone could tell - it was Real Money purchases only. However, the perk is going to be changed to the following:

On certain days, Zaldia can be groomed twice. When groomed for the second time, he offers 1 Diamond as a bonus.

It's only the card packs you can buy at any time - and the Joker packs with 3 bronze and 1 higher rarity (Which another run of the Joker packs will happen once again towards the end)

Same here. But then again I'm not willing to wake up at 1am to click a Pinata, then try to get back to sleep.
I wish they put Zaldia in a different promo.

Saddle Cloth
Saddle
Bridle
Polo Wraps
Ear Bonnet.

The shortest time span between clicks in this event is every 3 hours...and this only happens on later Piñatas.

Keeping that in mind - let's take a look at someone whose clicks are 3 hours apart:

1am - 4am - 7am - 10am - 1pm - 4pm -7pm -10pm

To me, the best course of action is:

Stay up until 10pm - get those clicks - then go to bed > Set your alarm for 3:50 am > When you wake up, make your 1am clicks, and 4am clicks, then go back to bed > Wake up later before 10am to get in your 7am click.

Of course, this might not work for everyone depending on their schedule - but it does help cut down on the 'losing sleep' factor.

That's why the perk is being changed. The majority of players voiced their disapproval of the perk and how ridiculous *being nice here with my words* we felt about it. The Howrse Team listened and opted to changed the perk. It's not exactly the best change (given how rare it seems that a Legendary does its action twice) but it is a lot better than needing to spend money to get a single diamond.

Is is possible to get Zaldia without spending any passes or opening any HOPs? Assuming I get the maximum amount of clicks every three hours, from friends, etc.)

No. Passes or saved HOPs are necessary if you want to obtain Zaldia through the piñata promotion.

"On certain days, Zaldia can be groomed twice. When groomed for the second time, he offers 1 Diamond as a bonus."

Does this mean Zaldia gives you 1 diamond?

No Pinata promo has ever been possible without the use of stored HOPs/Passes...and this one is no exception it seems:

If the promotion is 19 days long - without using Passes/HOPs (outside of objectives that require it - and on the assumption that extra points are not offered in HOPs) - one can only fully complete up to Pinata 16 by getting in all normal clicks. You'll get between a third to halfway done with 17 by the end.

Using nothing but Mace Blows (extra clicks) on the Final Full Day of the event - I came out to one needing to use around 113 Passes.

Again though, that's on just purchasing Mace Blows alone...It did not include using stored HOPs, or the off chance of double Mace Blows (6 for 1 pass instead of 3 for 1).

Once testing starts on that - I can get a different calculation on passes that will seem a lot more reasonable in the end (as depending on the Pinata - you'll get more points spending 1 pass on a HOP that you would if you spent that 1 pass on 3 Mace Blows.

Oh looked, it worked!

but again - you only get that 1 diamond IF Zaldia lets you groom him again in the same day.