Right, not sure how that would work if it’s streaming it. TrainerRoad expects you to have the file locally, which you do get when you purchase it from Sufferfest. I suspect you’ll just have to try and start pedalling and press play at the same time!
May be worth posting something on the TrainerRoad forum, the guys who develop it are good at getting back to you.
Edit: … Re the sweet spot question. Can be. For me though, generally a sweetspot one will be about 5 to 10 bpm lower but with a similar gradient across the duration of the interval than a similar threshold one. Though I don’t really pay it much attention other than as a rough indicator of tiredness and form ( I only really have a rough idea what my HR threshold is, it’s not something I bother noting or measuring.)