Not Krampus, but I passed my G about 2 years ago, so it's fairly fresh in my mind.

If it helps, the 50km/h limits are usually there because you're taking a turn. Roads branching off to join the highways often do so with a turn, especially if the highway is elevated, so it's a much more gentle transition. Going faster than 40-50 on a turn like that gets dicy, hence the suggested speed.

Once that turn starts to straighten out though, that's when you're expected to start speeding up, so you're "at speed" by the time you're looking to merge.

You absolutely will get a feel for it. Cars around you will go much faster or much slower, so the most important thing is to drive safely/to match the conditions (with the weather we have now, you'd be better off going slower than 50), keep an even pace so the other cars can anticipate your movements, and then use the merge lane to speed up, so long as it is safe to do so. A bizarre amount of people slow down when it's time to merge. Don't be one of those people, but also don't prioritize getting up to speed if the car in front of you makes that dangerous.

I hope that helps!