In answer to your questions:
1. AppThemes does not officially support third party plugins, so unfortunately we cannot guarantee that all will be compatible with our themes. Occasionally plugins are mentioned by members of the community as being used successfully, however this can change between versions, so ultimately you may not be able to rely on a plugin to definitely work without additional modification. For popular plugins specifically tested and confirmed to work, we often list them
here.
2. Our themes are not encrypted, so you are definitely free to modify them as you wish, in fact we encourage it! You can find useful developer documentation
here.
3. The $99 is the single theme price only and does not include child themes. The AppThemes club packages only include AppThemes created marketplace products, not products created by other marketplace developers. This is explained in more detail in the
pricing page FAQ. Thanks.