Fuel use in combat is, as you've indicated, dependant on a number of factors, another one being the # of units involved (eg. how many tanks have you in your Panzer Division). It'd be very difficult to give an accurate estimate prior to a combat. A simple rule of thumb is that the bigger the battle the greater the fuel usage which is probably as much as the commanders on the day had to go on as well. Telling the player that this particular combat will incur a fuel usage of 110 barrels would also be unrealistic and go against the ethos of the game.
A fuel warning indicator is an idea that I tossed around for a while and eventually binned. As there are going to be numerous instances of running low I didn't think the hassle of being continually spammed by 'Your fuel is running low' messages would offset the times when you would have forgotten to read your daily logs, logistic reports or checked the main screen fuel gauge. As fuel is the number one critical factor for the Germans you tend to take a strong interest in it's availability once you've played the game for a while.
The colour of the depots (main, FSB and PG HQ's) can give you a pretty good visual guide as to your current situation and are probably the easiest way to judge how far you can push things in any given turn, eg. if you've got less than one quota present at the start of a turn then you'll need to be paying attention to the fuel gauge. The PG tab down the bottom also has a quick visual indication (arrows point up, sideways, down, etc.) of how much fuel each PG has available.
So, nope, can't help you with either request but at least there's an explanation as to why.