Hiring a car in one country and returning it in another is very expensive. Unless you are returning to Switzerland hiring a car for this trip could be too expensive.
According to https://www.viamichelin.com/ Time: 06h28, Distance: 594 km including 516 km on Autobahn
That time assumes no stopping, I would add 10-25%
You would need Swiss and Austrian Vignettes to cover the Autobahn tolls. A car hired in Switzerland would already have a Swiss Vignette for 2018. You would have to buy an Austrian one at the border.
"Are the roads very curvey or steep if I decide to drive." - as you can see above, it is mostly Autobahn.
"I would like to stop halfway over night around the lakes, then continue to Hallstatt" - not sure which lakes you mean. Your route passes Bodensee, and Lindau is a good place to stop, but Bodensee is only about 1/3 of the way.
By train, look up times on the ÖBB (Austrian Railways) website: https://www.oebb.at/en/
This takes minimum 9 hours. But you are sitting in a comfortable train looking at the scenery. Salzburg is a good place to stop off for a night en route, but no lake there.
"How easy is it to get on and off the train ." - not sure I understand this question. Easier than getting in and out of a car, about as easy as getting in and out of a building. Sometimes one or two steps, sometimes none.