re: When to pay off the mortgage

I say if you have the money pay it off today! Besides a credit card that's used for household purchases (that's paid off monthly), I owe nothing to no one. I own many properties this way. It's less paperwork and stress. For those who would argue with this and want to talk about the delta between saving the capital vs deduction on taxes vs the interest accrued on the mortgage.
I would recommend you read The Millionaire next door by Dr. Thomas Stanley. This is a survey of what the wealthy actually do - not one of them has anything good to say about debt. Additionally, the large majority of them own(ed) their house outright.
My advice is: Do what wealthy people do. Not what your broke friends are doing.
Once it's paid off, continue to pay the payment into a separate account and invest it once mkts level out.