I am a Canadian citizen. I will leave about 70 days into the Schengen visa given 90 days upon my arrival in Italy.
As a Canadian citizen you don't need a visa for short visits to the Schengen area, and you won't get one. You can enter visa-free, which means "without a visa".
I will go to Thailand for 12 days, and then return into England, with the wish for the chances of getting 90 days again (the stamp is for some reason in the middle of my otherwise blank passport you don't open to it directly.)
You don't "get 90 days". There's no concept of a 90-day period restarting. No matter when you enter and exit the area, the rule is always the same: You can be in the Schengen area for 90 days in a rolling 180-day period. Or in other words: At any given point in time you must be able to point to 90 dates within the last 180 days where you were not in the Schengen area.
With the plans you outline, this means that once you have been inside the area for 70 days contiguously, you have 20 days of allowed stay left. It will take you 90 days outside the area to accumulate enough out-of-Schengen days qualify for a subsequent stay of more then 20 days.
Can I get a renewal done if I get a letter from a non-profit community saying I am wanting to learn and volunteer more?
You may be able to get a long-stay visa from Italy if they think your purpose for going is one they want to support. You should talk to your host organization to hear if they're able to sponsor you for such a visa, based on local Italian law.
If I re-enter Europe into England, how will they know in Italy when I entered if I then take the train from England to either Italy or Portugal? I was thinking to do this instead of fly to get around the stamp into Schengen.
Note that England is not in the Schengen area.
If you take the train between England and the continent, you will go through the external Schengen border checks at the station where you board the train. During those checks you will get a dated entry stamp in your passport, just like if you enter the Schengen area through an airport (or land border crossing).