Re your second paragraph. Yes, it is nigh on impossible to watch both the defensive line and the oncoming wide player especially as the linesman in all probability will be running backwards, a point that shouldn't go unnoticed that running backwards isn't natural; if it was we would have reversable feet.
My point is, that football is not solely officiated with the eyes but with the ears also. By which I mean that as a linesman in the scenario described you size up the situation and concentrate on the likely outcome: is the ball about to be crossed? or is the wide player about to be tackled? In the first instance just concentrate on the cross going in and look for a possible offside, listen for the sound of the foot making contact with the ball and base any offside decision on that.
In the second instance just watch for any foul tackle that may occur. I'll just add that before anyone offers the speed of sound as a difference of opinion, football is not an exact science and once we get into the realms of the minutiae of it, then it's time to consign it's sorry arse to the bin. Just my opinion of course.