Greetings!
I hope I'm able to help. I'm about as clear as mud a lot of the time. The sprites for the individual candy hearts can be a pain since there are three of them to keep track of. When you edit the file in Tinker, do the sprite names show up as the original sprite names or the ones you changed them to?
I don't know if my tutorial is one of the ones that you have been using or not, and I don't want to repeat a bunch of stuff you've already read/know. However, I think that starting from the beginning and looking through the file is often the best method for pinpointing a sprite issue. The type of issue you are describing is a sprite name issue, and those are very common. So no worries, this is also a problem that is quite fixable with a bit of patience.
LNZ Pro is what I use first. You need to change sprite names in every section except for "WAV" since that section is just the sounds themselves. That means you will change sprite names in SPR (or FLM and FLH if you are in Petz 3 or 4). You will change sprites in the "TXT," "String List," and RCData sections also. Basically, if you see "TBox_H1" you are replacing it with your new box sprite name, and if you see "Trea_H1" you are replacing it with your new treat sprite name. "Trea_H2" and "Trea_H3" will get the new sprite names for treat 2 and 3.
In RCData you will rename the sprites and give your toy file a new name with the same number of characters as the original file name (Candy Hearts). So for my example I will call my new toy "Sandy Hearts." "ECRose's Example Hearts" has too many characters. Spaces count as characters so if I wanted to have my "prefix" in the toy name I would have to call it "ECSandHearts" to have the same number of characters. You will put in this new toy name in the String List section where "Candy Hearts" appears as well.
Be sure to change the ID number for both the box (TBox_H1) and the treat (Trea_H1). I am presuming that you know all of the places in LNZ Pro that need to have the sprite names changed, but it is always worth looking at all of those places again to see if you missed one. Remember, in RCdata you change the name of the treat "1" sprite and give it an ID number. In the sprite (or FLM and FLH) sections there are the "2 and 3" treats.
So let's say I rename my treat sprite "Sand_H1." I will change "Trea_H1" to "Sand_H1" in RCData. Anywhere that "Trea_H1" comes up will get the same treatment. I will put "Sand_H2" and "Sand_H3" where "Trea_H2" and "Trea_H3" appear.
Save your file as "Sandy Hearts" in LNZ Pro with its spiffy new ID numbers and changed sprites. Close the file. Open the file in your hex editor. If your editor has the "find" feature, use that and type in the word "download." This option is usually under "Edit." If your find feature has the option to search by ASCII or Hex, use ASCII.
When you hit search you will be taken right where you need to go in Petz 4 and 5. You have to click "find next" a few more times for Petz 3 files. Trust me, you will know when you get to the right spot. It is when the file stops looking like mush.
The spot you find yourself is at the "download sprite" section. There are numerous paths here with the original game sprites. This part of the file tells the game where to look for all the filmstrips and information. Go through this section very slowly and carefully. Replace the TBox sprites with your new box sprite. Replace "Trea_H1" with "Sand_H1" (or whatever you are using). "Trea_H2" and "Trea_H3" will get "Sand_H2" and "Sand_H3" respectively. Work your way down to where it says AV type info. You will see a path right above "AV type info."
\Resource\Toyz\Candy Hearts.toy
Replace "Candy Hearts" with "Sandy Hearts" or whatever your new toy is called. It needs to be the same number of characters as Candy Hearts. No more. No less.
Once all of the sprites are renamed, save and close the file. Open the file in Tinker. You will have a list of sprites to edit. Do the names all match the new sprite names that you gave them? Do they all open? If you said yes to these questions - awesome! If not, you will need to look in LNZ Pro and your Hex editor again to see if you missed any sprites. Remember to only have the file open in one program at a time.
I hope this is helpful. If it is unclear at all just say so. I know this can be a bit confusing. I can attach screenshots if you need them.
Happy Hexing,
~ECRose