Welcome to SitePoint , Oceanic,
you have got to love the mess of imperceptible terms in creative industries. You have to remember many of them are CONCEPTUAL rather than concrete. In other words pixels dont exist, as such they have no actual real world translatable measurement.
a 2"X2" screen could display 100X100 pixels or 10000 X10000 pixels ( even non square pixels 5000 X 10000 happen)
Resolution: is the dimension in pixel , independent of anything else (like actual size) , this can be confusing because in PRINT DESIGN resolution is dependent on size.
Pixel Density: is the equivalent term for when design . So if if one device has a pix dens of 72px/in and another 144px/in you know that an image with the same resolution will be twice as large (or 4 times the area as you put it) in the first device ( size is inversely proportional with pix dens)
Aspect ratio is is the Horz./Vert pixels measurement.
with that primer your answer are as follows:
1) You don't. as thee is no such thing as true pixels. What you do have is aspect ratio, and pix dens.
2) AFIK, the device width (in the mind of the device) equals the # of pixels it can display( what you are calling the 'true pixels') so a device that can display 1260px will assume it's a monitor 1260px wide even if those 1260px are crammed into 4" instead of 21". (thats the rub)
3) No. Hardware is physical. You can have less than the max , but you cant ever have more than the max. This just a law of physics.
4) in four questions we have addressed this 3 times now, lol. there is no defined physical size or even distance between pixels. think of it as a conceptual measurement for 1 point of light. the actual size is determined by a bunch of factors completely irrelevant to GOOD responsive design practices.