Determine total current in a darlington transistor
Start with the information that is given for the current gain \$h_{FE}\$, you should be able to extract a "ballpark figure" from the datasheet:



Note how that information is quite "basic", all that the manufacturer guarantees is that \$h_{FE}\$ will be larger than 1000.

So you will not be able to find an exact value because \$h_{FE}\$ will never have an exact value, unless you take a transistor and measure it, then you have that transistor's \$h_{FE}\$ at the conditions at which you measured it. The next transistor in the box might have a completey different \$h_{FE}\$!

So stop trying to find an exact value for beta. What engineers do is that they design the circuit around this transistor such that it will work reliably for \$h_{FE}\$ = 1000 and higher values. (And most engineers would also add some margin to that so would design for \$h_{FE}\$ > 500 just to be sure).

The resistors don't really work as a a voltage divider as there are also the base-emitters in parallel with these resistors. The resistors are there to make sure the transistors switch off when no input current (at the base) is applied. Since the TIP120 is designed for switching applications, this makes sense. Without these resistors it would take longer to discharge the base of the "big" transistor when we want to switch it off.