The second yeast will eat the dead yeast. Even if both yeasts survive, it will be like a large starter, quicker fermentation.
No difference in taste..
The yeast rehydrated in warm water would die in hot wort. If the temp
was over 90F after adding yeast, it is probably toast.
(This is the type of emergency that causes most brewers to keep
an extra packet of dry yeast in the freezer)
BUT,,. Unless your sanitaion is excellent, I would not run a heater in wort
that has no viable yeast in it. I would keep it sealed in the fermentor and find room in the fridge or in a bucket, covered, with ice around it and prevent
wild stuff from growing.
What kind of yeast are you using?
Unless your fermentation area is less than 68F, I would not bother with the heater.
BTW..It WILL be BeeR!!!!