This is just how status codes (and virtually all measures) are seen by the monitor and most of AppMon. Even though we can recognize it as a code AppMon sees it as any other number which is why it includes that decimal. I'm not aware of any way to change this.
This isn't really related to the plugin, this is how the server handles the measures. If you were to use the standard email action or chart this measure you would see the decimal as well. I have never seen any way to round off the decimal or make it an integer or anything like that.
If you find this important and no one else has any ideas you can open an RFE/idea in the AppMon and UEM Product Ideas forum.
Not that I'm aware of, the plugin is just using the values passed by the server - the plugin would either need to modify this value to show as an integer or string (without the decimal) or you'd have to be able to somehow use regex on the value in the body of the message to remove the decimal. Neither of which is possible as far as I know - modiciation of the plugin would likely be necesary.
I cannot, this would not be a simple change and the extended email plugin is especially complicated given all of the features and options it has. I am not familiar enough with it to be able to make such a change or even recommend how to get started on that. My comment was more of a "theoretically this would be possible" as opposed to a practical solution to the functionality you want.
Is there any reason the decimal is such an issue? You can customize the body of the email to clarify that it is a status code if the worry is that there could be some confusion.