H Roger,
>>However, when I created a new add-in project or open an existing one using the IDE menu and debugged it, a new instance of the host application >>is launched.
I have confirmed this and will ask the VSTA team to improve this sample for the next version of the VSTA SDK.
Your intuition is also correct. In the meantime, I will provide you a sample that implements addin project seamless debugging along with the macro project seamless debugging.
Regards,
Gary