Went the Merchant Navy route myself, after 5 years with Shell Oil I walked away with a 2nd mates ticket and enough money (thanks in part to the tax advantages of being off-shore ) to pay for flight training. In retrospect it certainly proved the better option than that of sitting out a recession.
The MN isn't a bad job while you're young and without family obligations but the life style would be very difficult to put up with once the future wife and kids arrive on the scene. Suggest that you take the ATPL self study while at sea, when the time for the big push arrives it really does help you hit the ground running if the paperwork side is in the bag.
After experiencing both MN and airline life styles I can attest to the fact that there's little contest between which is the better. Using the MN as a means to an end is certainly a viable option with the only negative in your case being the two year commitment after training. Best of luck, you'll certainly not be the first ( nor even the second) to take this route.