The close polls and seat projections are making a minority government after October 19th almost assured, and the recent resurgence in Conservative polling numbers means that Harper could very well continue to form government after the election. But with a minority, and with neither the Liberals nor the NDP keen to support him (Mulcair used the phrase "snowball's chance in hell"), it starts to look doubtful that the government would survive the throne speech.
So what then? Would it mean a coalition? Would it mean another election? Well, first of all let's remember that under our constitutional conventions, the incumbent government gets the first chance to form a government because they are already the government. Government (by which we mean cabinet) doesn't actually dissolve when there's an election while parliament dissolves, the government stays in place to keep the joint running. We call it the "caretaker convention," which means the lights are on and the mail still goes out, but they just can't make major policy decisions. That government stays in place until the Prime Minister chooses to resign.
Why is this important? Because the Governor General doesn't get to pick who gets the first chance to form government, and that even if Harper gets the most seats but not enough to get a majority, he still has to let Harper have a chance to test the confidence of the Commons because he is already head of the government. Elizabeth May might think that she can call up Rideau Hall on election night and say "whoa, let the other parties try and form a government first," but it doesn't work that way.
So, carrying along this hypothetical situation and Harper wins a minority and tests the Commons with a Throne Speech, and surprise, he gets defeated. (This isn't of course, guaranteed one of the parties could abstain from voting, or enough MPs could conveniently be absent in order to allow it to pass if they don't want to defeat him on the Throne Speech something that has never been done before in Canadian history and they want to defeat him on another bill or motion). What then?
This is where the Governor General's role becomes much more prominent, where the comparisons between what happened in 2008 will really come to the fore. It's also where our conventions start getting into the weeds a bit more. If Harper were to be defeated on the Throne Speech, or his first test of confidence post-Speech, he could advise the GG that he wants to call another election. At this point, the GG has to make a decision whether to accept that advice, or to reject it. If he rejects it, the convention is that the Prime Minister has to resign, and the GG can see if anyone else is capable of testing the confidence of the Commons. And it's at this point where the second-and-third place parties will need to work out some kind of arrangement.
All parties have stated that they don't want a formal coalition including May and the Greens, for whom she said she would work best holding the governing party to account from the opposition benches. (One would have thought it's more effective to get a seat at cabinet, like the environment portfolio, but whatever). That means that there is going to have to be some kind of agreement worked out that would allow the second-place party to be able to assure the GG that they can maintain the confidence of the Commons, and that may involve the third-place party, whichever that may be, to swallow their pride if they want Harper gone while avoiding another election so soon. If they can't come to some kind of an agreement, then it's back to the hustings we go.
Where the comparisons between this theoretical post-October 19th scenario and 2008 come into play are vastly different for a number of reasons. The seat distribution in 2008 was 143 Conservative, 77 Liberal, 49 Bloc and 37 NDP with 2 independents. According to the ThreeHundredEight.com seat projection as of press time, we're looking at 128 Conservative, 110 Liberal and 98 NDP, with 1 Bloc and 1 Green seat.
Harper made a lot of hay in 2008 that the proposed Liberal-NDP coalition which would still have fewer seats than the Conservatives had was dependent on Bloc support, hence "giving a veto to separatists." The Bloc are unlikely to be a factor after October 19th and a theoretical Liberal-NDP agreement post-October 19th would not face that disadvantage in terms of seats. Add to that, in 2008, Stéphane Dion had already committed to resigning as leader, hence the coalition would be led by someone on his way out with a huge leadership question mark hanging over it. It made for poor optics. This is unlikely to be the case after October 19th, as Trudeau would likely not resign after increasing his party's seat count dramatically, and it's likely that Mulcair would be given a second chance (unless the knives come out, which nobody should rule out). With all of these factors in mind, it's not too surprising that Her Excellency Michaëlle Jean decided to grant Harper his prorogation. If the coalition was serious, it could have attempted to vote no confidence when the Commons returned. It collapsed, making Jean's decision all the more prescient.
All of this remains theoretical, of course. There remains the theory that Harper may delay reconstituting Parliament until the New Year in order to hold onto power, which may turn public opinion against him. The unofficial convention, per the opinion of Governors General past is that they are reticent to go to an election less than six months after the last one, but keeping Parliament unconstituted for that long is dangerous given the caretaker conventions and the possibility of a crisis emerging that requires parliament's attention. Suffice to say, there are plenty of options to explore after October 19th, but a snap election is less likely.