Welcome to the forum. Actually your questions can be extremely simple. Mostly, one is dealing with VARIANCE, so it can be extremely (more normally) complex. In that sense, normally one is concerned with what one is ready, willing and able to get.
Once you know the parameters of the 1.9 TDI engine, you can operated it in ways to get literally whatever mileages (RANGE) you want, given your conditions. My ranges have been from 44 to 62 mpg (175,000 miles) . There are loads of folks who really don't even know what the specifications ARE, let alone understand how to use them to get maximum fuel mileages or whatever. Whether you want to operate in those manners are different questions and can be in perpetual flux.
So yes to answer your question, 50 mpg in a MK IV Golf TDI 5 speed manual is do able.
No, I can get 50 mpg (range of 48-52) in a daily 54 miles R/T commute taking anywhere from 45 min to 1.5 hour for a 27 miles leg, for 11 actual commute days (average). I can also get 50 mpg @ 90 mph, on a road trip. I have routinely gone 75 mph on the highway (road trips) and posted 59 mpg. I have yet to do a longer time perpetual operations at 55 to 65 mph, but I swag it would be more along the lines of 60 to 65 mpg. Again this is on a heavier car than your Golf TDI Mk IV.
So for a snap shot, I filled yesterday after 3 weeks and not even caring about mpg, app 500 miles 10 gals for 49.7 mpg. Pretty easy to do around here, especially if you keep it under 95 mph.