Hi and welcome to railcc!
With the new rule the trip with this night train still would count as only one travel day; however if you want to take another train _after_ the night train (on the day of arrival) you would need to use another travel day.
The new rule kind of reverses the existing 1900 rule: with the new rule you can now make as many journeys as you want on day A (day of departure), then take a night train and travel with the night train to your destination without having to use another travel day on day B (day of arrival).
As far as I know 2019 will be a transition period and the 1900 rule will also still be accepted. I dont know (yet) whether you can use both rules with the same Interrail pass/during the same trip but I assume that this is the case.
All trains between Chur and St Moritz are operated by Rhätische Bahn (RhB) and Interrail is valid: rail.cc/en/interrail-train-company/rhaetische-bahn/63
"TRN" in the Rail Planner App translates to "Interregio" (= "IR) in the "real" world (in this case).
No reservation needed; if you travel from Chur sit on the right hand side of the train (beautiful views on both sides but slight advantages on the right; also the view of Landwasser Viaduct is much better on the right).
Also, if you have the time, go at least to Ospizio Bernina (or better Alp Grüm or even all the way to Tirano) from St Moritz! The Bernina Railway is a beautiful, spectacular line and easily doable as a day or half day trip from St Moritz.
It is also operated by RhB, no reservation needed.
I should also mention that on both lines additional trains with panoramic coaches operate (Bernina Express = BEX and Glacier Express = GEX); however a reservation is required. They run on the same lines so in my opinion it is not worth to pay extra.