You did right when you bought the Toyota belt. I have tried the Gates belt before, and besides being stiffer and thicker than the OEM (which makes it harder to stay in place) the marks on that belt were off. As you point out, there is a way to put that belt on that seems right but it isn't. You are off by one tooth. You gotta almost stretch the belt to get it on that back sprocket the right way.
The Gates belt had the marks off ever so slightly but it was enough that there was NO WAY you could get it onto the engine! No way, no how that line could fall into the sprocket. I took that belt back to NAPA and bought an OEM belt which went on fine.
I've changed the timing belt on 1MZ-FE engines about 10 times. I've got it down now where I can get the old one off in about an hour which isn't bad.
On that idler pulley though (there are two of them), my upper one failed at about 150,000 miles. It made a knocking sound at a certain RPM that sounded like rod knock. I didn't replace them at the first timing belt change, so I had to change out the replacement a little early. No big deal but I would suggest (as does my local dealer) that you change the pulleys every time. It's an added expense for sure but they do fail. It gets hot under that cover.