Strandafrit, strandopgang and strandslag are synonyms. Strandslagen is the plural form of strandslag.

Beach entrance is a loose translation to English. It then represents the entrance / 'cutting' through the dunes itself, not the path. So according to the One feature, one OSM element principle of OSM it should be tagged separately. 

Municipalities sometimes name the paths after the strandslagen, but they could also be entirely different. In the case of Katwijk, I'm not really sure whether the name-tag on the paths is correct. They may be part of the road Boulevard or Boulevard Zeezijde.