You have a decent amount of flexibility in what to do over that 1.5 hours. Yes, you can check-in, eat, leave, then head to the viewing spot. You just need your dessert party wristband to enter the plaza garden area, you don't need a CM escort.
I will start by saying there really aren't any bad viewing spots in the garden, just subtle differences. Typically, the garden will start to 'fill in' as you head towards show time. I am a bit more anal about my spot than what I should be - I prefer back left on the rail - so I try to balance getting there too early vs not wanting to be there late when it is more full and just having to 'fill in' spots. But where to peg that timing is really tough to be specific as it can vary on any night.
I'm 'probably' going to check-in 9:30'ish, have a quick look at the desserts, bathroom break, then head to the garden.
Last year, I 'watched' the first parade from the garden, so I checked in more like 9, had a quick bite, then went to the garden. It is not a good view at all, but you are close enough to see some things and hear the audio, so it's OK for a quick parade fix (we then watch the second parade for real later on). This was an OK plan for us, but did mean a lot of time in the garden, so not exactly ideal for how we do things.