Thank you for reporting these issues!
Kanryu: I have just added HHO to the card database. Please have a look:
https://deckstats.net/cards/Happy+Holidays/Tetra: I have fixed the card number for
Aether Spellbomb (and some other cards with an AE in the name).
However, I am not sure what I should do about your second point. I looked into this a bit further and it seems like the collector numbers start being offset from
Howling Mine, which is #290 on the card but #289 in our database. The card before that is
Grinning Totem, which is #288 both on the card and in our database. So it looks like there was actually an issue with the numbers on the printed cards, with #289 missing. Maybe there was supposed to be another card there that got removed before the set was released?
I could of course change the numbers in our database to match the printed cards but then our data would no longer match with Gatherer/Scryfall. So that might cause other problems later on. I will have a bit a think about this and get back to you later.