Can I suggest that the OP has a read of this thread, in particular post #18 which will be useful.

No idea why the airport website is directing you to go to transfer desk - you can go to arrivals directly but if you want to get rid of your hand luggage in the lockers a quick trip to the lounge might be useful.

Immigration queues can be variable but if you are travelling in J you have access to the arrivals lounge with its own dedicated immigration officer. Additionally with 12 hours layover you could hang around in the lounge for a hour or two to avoid any busy period before leaving. Again, returning to the airport before the busy period will make immigration a complete breeze.

Doha is a small place and the number of attractions in the city are limited, so 10 hours to explore the city will be more than generous. However also worth noting that there has been at least one report on here of immigration refusing a very short layover (just an hour or two), but with more than 5 hours I don't think you will have any issue being able to exit the airport to explore the city.

Enjoy!