So do I begin to understand the Canadian government correctly?
Barring the unlikely, is he in office until parliament is formed again in 2019—if a different party then gains the majority?
Is there any realistic chance of elections being called earlier?
That’s correct. He could call an early election but that is unlikely unless he feels scandals are approaching - it often backfires as Alberta saw last year and Ontario saw in 1990. It worked for Chretien in 1997 and 2000 as there was no united opposition.
Note that the two main opposition parties do not have a permanent leader right now (although the NDP is in total disarray with no candidates for the job). He is still leading in the polls, but only by about 10 points rather than 30 points. If he trails against leaderless parties, that is a big problem.