I have found that the various players and platforms have their own quirks. I have found that Garmin, Android (WhereYouGo) and iPhone all vary on how they handle the same cartridge. I can't claim to know all the different commands that Garmin does not support, but I can offer a way to handle character interaction. I use "commands" within each character (or item) which provide a defined method of interaction, and have found that to be quite robust across the different platforms.
Unless you have access to all platforms for testing, I suggest that when you release your Wherigo cache page you add a note such as;
"You will need some form of Wherigo player to find this cache.
The cartridge was created using the Urwigo builder, and has been tested on the following devices (if you let me know the platform you use, I can expand this listing);
* Urwigo test platform
* Android WhereYouGo player
* iPhone
* Garmin Oregon
Please let me know in your log which platform you have used to play the cartridge."