You must be a really strong fellow! I didn't even think of pushing it all the way down like you did. I'll give it a try next time I remove the seat. BTW, you don't absolutely need to cut a notch in the seat. Using a shorter bolt to hold the brackets would have solved the issue... It's definitely possible to install that tank without having to cut anything on your bike but the side covers.
On a side note, I went for my first 30 miles ride with the new tank this morning and I can tell you that it feels like it was made for this bike. No wiggling, buzzing, rattling or discomfort. I was really glad to see the tank almost still topped off went I arrived at work!