Funny you should mention Timatic, because there's something that occasionally appears in Timatic which causes airlines to absolutely insist on an onward ticket.
Visitors not holding return/onward tickets could be refused entry.
Which is further explained as:
Immigration authorities may request visitors and transit passengers to prove that they will depart from the country within the prescribed period, by showing a return or onward ticket to their next international destination.
Unless stated otherwise, return/onward ticket is defined as:
a. International airline ticket (i.e. any types of airline tickets, reservation confirmation, booking code etc.); or
b. Evidence of departing from the country by other means of
transportation (e.g. confirmation of joining a cruise, train,
bus or ferry tickets, proof of departing by private boat or
And, when we enter your itinerary into Timatic, these magic words appear.
Of course, because the airline has to transport you back, and may be fined, if you're refused entry, they take this warning very seriously, and will almost always deny boarding or require you to buy an onward ticket on the spot.
Speaking of which, the cheapest onward air tickets are likely to be to the United States (e.g. MEX-IAD), though of course nothing requires you to board the plane or travel there... If you have an idea of where you'll be going next, you can simply book that onward ticket.