There is no sleeper train that run anywhere near that route. There aren’t really any overnight (non sleeper) trains either - you will likely end up on short local trains, with long stops somewhere in the middle of the night.
It’s possible that you could work in a sleeper train if you take a longer route. For example, from Paris to Nice, then work your way back up via Milan to Tirano (the Italian end of the Bernina) but that only really works if you have plans to visit the south of France and / or northern Italy.
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To your second question - yes, there are frequent (hourly) trains along both routes, usually needing a change or two. They don’t need reservation and they’re very good quality trains.
But also, you don’t need a reservation on the Bernina Express, as long as you travel in the unreserved cars. This isn’t true on the Glacier Express - on that one, the whole train is compulsory reservation, so it’s best to stick to the regional trains along the same route.