2000mAh, 20000 is beyond anything you will see in normal size cells today. You might get a drill battery or laptop with that but it will have possibly 20 or so of those cells to pull that off.
If the console runs off 12V and the screen does too (I assume this is the typical use a screen aimed at a car/caravan approach rather than something custom -- not so many screens come with nice inputs for old consoles) then it makes sense to combine it. The current draw of the 3.3V rail is 1.5A according to that link which is considerable but not impossible by any means. Still you can easily drop down if you don't want to run parallel rails or multiple sets of cells (you don't).
These days you have many options when dealing with power but the classic option of make the voltage you need with cells and then drop whatever else is still the best method when you need any kind of power -- trying to convert voltages to greater ones is doable enough but DC and already fairly low voltages is not a recipe to make that easier. You mentioned you have a fairly nice regulator which is nice, however for a classic regulator then whatever you don't use get puts out as heat, or if you prefer the
[email protected] is fine but you then have the better part 10V @ 1.5A going out as heat which is a lot of heat indeed, a more sensible conversion method will then be needed.
A nice start for you might be watching the following series, it covers much. The second video specifically deals with DC voltages
For power calculations specifically then it is actually somewhat tricky. That said the first blush method is to assume you have full capacity in the cells (you are told not to discharge your devices fully too often for a reason), the voltage will hold throughout it (batteries output less voltage) and will always be able to deliver the current you need (there is a concept called internal resistance that ultimately limits things but let us not go too into that). That allows you to use the classic power = voltage x current (P=IV) and energy = power x time = ITV equations to get in the ballpark, it is certainly never going to be more than that unless your cells are higher rated than they claim. It is why the link above took the effort to convert it to Watts (a unit of power) as you can add things there and not have to worry about too much.
To that end as you were off by a factor of 10 above then I will take the 5-7 hours and divide it by 10 to get half an hour to .7 of an hour which sounds about right, though actually as the two cells will not make 12V when combined it will not even power it unless you added some voltage increasing stuff in there which is not the most efficient thing to do (if you even can) and that only drops it further.