I book one way at a time with all our travel to Europe; there has never been an issue with one-way flights.
But when we flew to New Zealand from Australia, the Emirates agent at check-in asked to see proof that we had a flight out (which we did). I knew about that requirement and had a copy of our return ticket (New Zealand to Australia) so it was not a problem.
For your trip insurance, assuming yo do not want to purchase an annual policy, you could just enter the 60-day return date, and the trip price you have paid so far (the one-way flight and any other non-refundable payments) for the trip cost. You can amend the trip cost, and the actual dates, later, up to the date of travel (or the day before) but not once the trip has started. I do this way myself, as I have often not figured out the return date at the time I book the outbound flight. Just make sure it is all correct before you depart, or you could be denied coverage.