1. the bike has probably been sitting in previous owner's garage for 6 years (based on last inspection sticker) and only changed oil every year.
Only every year? Or the only maintenance done ever has been oil? As for oil, I change mine far less often than annually- it depends on mileage and type of oil (I use true synthetic). First change was with dino at 900mi, then with synthetic at 2,631, 9,653, 21,500, 36,117, 43,428. Lots of opinions on that, of course.
What maintenance should I do as winter projects? there are a bunch listed in the manual, but not sure if they are absolutely needed.
Depends on mileage and what was done and when. Report that back and people can better answer your question.
Want to postpone something to valve check time.
What is the mileage?
I can say, I waited FAR too long to do my first valve check. Some were WAY off. I was fortunate there was (as far as I can tell) no damage.
2. the tires are Michelin, look brand new but 8~9 years old. should I replace them or can get a few hundred mile out of them?
Impossible to know without seeing the wear pattern and feeling the rubber. Even if wear is OK, tires get harder and less safe over time, especially depending on where it is stored and weather/sun/etc. I am going to guess they should probably be replaced for safety reasons.
3. I haven't been riding for 7 years, plan to get back into this hobby. I know this is a big bike. any advice on how to safely get back into riding (e.g. what skills to practice, which videos could be helpful)? I am considering do back to MSF course again next year
Take it slow. Wear all safety equipment all the time- full faced helmet, armored jacket, heavy long pants, proper boots and gloves. Taking a course is not a bad idea. Always drive defensively- assume nobody can see you and everyone is out to run into you. Always be alert. And don't feel you have anything to prove to anyone.
4. I plan to own this bike for quite a while, how many years or miles of longevity should I expect
Properly cared for, people are reporting 100,000 150,000 and even higher mileage with no major issues.