Yes, I have Ryan as a non-zero win chance. Because of a complicated thing called "everything explodes". Essentially, Gary Johnson takes one state, really any state, but the bigger the more likely this happens. And the more states the more likely. But I think he only has any shot in one or two states at most. Anyways, then he 'blows up' the electoral map in such a way that there's no winner, as in no one gets to 270, and the House decides the next president, they choose Ryan. I think that percentage is lower than 5, but non zero so i put it as 5.
They (the House) cannot do that.
Because of a not-so-complicated thing called "the actual text of the Constitution.
If your scenario happens,
The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed; and if no person have such majority,
the House does not have an open election of whoever they like. Rather,
then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President.
which in this case would mean they have to pick from Trump OR Clinton OR Johnson.
You would need to get thoroughly bizarre and invoke this:
If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
for Ryan to somehow be selected by the House. That however would require:
1. A tie or 3 people getting electoral votes; AND,
2. The House being "unable" to select a President; AND,
3. The Senate being "unable" to select a Vice-President; AND,
4. Paul Ryan winning re-election (which is not guaranteed at this point); AND,
5. Neither the House nor the Senate deciding to bypass Ryan whenever they feel like it by "suddenly" agreeing on a President or Vice-President
At which point you may as well just skip to whoever is elected, along with their running mate, not being physically or mentally capable of serving on inauguration day, and Ryan being re-elected, after being retained as Speaker, in which case he would outright become President without any need for a vote. (Though he could turn it down if he felt like it.)
While theoretically that is a non-zero chance, it is more obscure trivia than a betting line.