Ok, I'll bite on this one.
Whilst I do agree with the other points you make regarding belief and respecting views of others, this example doesn't help support that arguement.
The reasoning behind this is because 1+1 = 3 can be proven physically, in that you can take one object, and another identical object and prove you now have two identical objects. At best, a person who holds to the belief 1+1=3 has done is substitute the renamed the definition of 2 with a new word, that happens to have another meaning to the rest of us.
It is a case that a lot of misunderstandings come down to this issue, whereby people's definition of a word varies, such as the word "God".