I've felt the hoses and it feels pretty solid. Maybe not as solid as ice, but not slushy by any means.

Auto AF will be fine in 50/50 concentration. Just make sure it gets collected in the spring. Its bad for the lakes/environment.

How much gas is everyone leaving in their tanks? For years, I filled the tanks up, put in the stabil and ran around for awhile. The last several years I have very little gas in the boat because of how gas breaks down and the importance with my motors of having good gas. Which is the right method?

Also, do you need to change spark plugs every season? If so, is that something that should be done in the spring or does it matter?

Take out 4 plugs, 3 hoses, done in less than 5 minutes, cost $0.

I have always ran my fuel down low 1/4 or less and treated that per my dealer, 9 years no problems... I change my plugs every year in the spring after running it on the hose first, not sure if it is necessary or not, but I do it anyway...

You should be able to go 300 hours on plugs. I've seen plugs at 500 hours that look like new. Today's newer motors come with platinum-tipped plugs that really should never need changed unless you have a problem with one.

Even if you get all the water out of your engine and there is no risk of damage from freezing I still like to run anti-freeze thru the system because I feel it protects the internal surfaces from corrosion and it helps keep your impeller and other seals lubricated for the beginning of next seasons start up.

For what it's worth, 27 years of winterizing boats - Drain Block ( make sure water comes out, they will plug with sand & rust) Drain Manifolds, ( same caution as block) Drain oil coolers, 5 gal bucket with 50/50 auto AF, start engine, suck up antifreeze until it comes out exaust. ( catch the discharged AF), Done. In the spring, fire-up and catch all the green and discard properly. I have done a lot of boats this way, has not cost me a engine yet.
Larry's Boat & Aircraft Repair
High deserts of utah (It gets real cold)

If you don't drain the water first, you don't know what kind of mix you are getting.

I agree 100%
In the spring I install the transom plug and remove the exhaust and blobk plugs and drain the antifreese into the bilge go outside with my 5 gallon bucket and remove the transom plug and catch all the antifreese and reuse it next winter.
If you have a auto bilge pump disconect it or it will go off with all that antifreese in the bilge

I remember winterizing....that sucks not only for the effort, but knowing your baby is taking a nap for 7 months.