Do you have to click the confirm at exactly the top of the hour at 0 seconds as well?
No you don't, and yes you do. Basically, you have to keep submitting until you see the "success" message. If you see an error message then the server is too busy, try again. If you see the "submission limit reached", then you have to keep trying or you missed the hour mark and have to wait another hour. As long as it says successful, it has been submitted and you just need to wait until viabtc mines the next block. Also, assuming your tx was in a queue short enough to be included. I have seen it been included in the second block mined. Guess lots of people lining up to use viabtc these days.
Just as expected which there would be lots of people who do wait for the next hour if the limit has been reached on a particular hour even myself do this thing which I do make timer on when to go in since I do know they hourly reset. Limits do easily be reached up in no time which signifies theres lot of people who are waiting for this service. This transaction accelerator is working and ive been using this to push some of my transactions which do confirm too long. As long it wont go below 100 sats/byte then this accelerator wont deny your tx hash.