In regards to the $750,000 limit, yes, refinanced loans still use the rules based on the date of the original debt.  So you can still deduct interest for loan amounts up to $1,000,000 of acquisition debt.

However, only interest for money that was used to buy, build, or substantially improve the home will be deductible.  Starting in 2018 (tax returns filed early in 2019) you can no longer deduct mortgage interest for money that was used for other purposes.  It does not matter when the loan originated.