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I'll respond to the Landsknechts later. As for William, there were several in the House of Orange-Nassau. The first was the Prince of Orange who helped lead the initial revolt in the 1560s and who was assasinated in 1584 at Delft. He is the Prince of Orange who was the acknowledged leader of the Sea Beggers who captured Brill in the early 1570s, and for whom the orange, white and blue "prince's flag" was named. Maurice of Nassau was his son.
The William you are thinking of is William II of Orange-Nassau (William III of England) who IIRC was the son of Frederick Henry of Nassau, another son of William I of Orange.
I can't find my Osprey Landsknects (the more recent version with the battle picture on the cover) so let's leave it at this.
Whether you believe Landsknects participated in the French Wars of Religion, the Seven Years War of the North, the conflict between Poland and Danzig in the 1570s, the early stages of the 80YW and other conflicts depends on whether you consider the German pike and shot mercenaries that served in those conflicts to be Landsknects or not. The term is used by sources to describe such German mercenaries in actions as late as Ivry. Of course it should be remembered that the term "tercio" continued to be applied to Spanish foot units long after their formation in the 1530s, when both organization and tactics had changed dramatically (just as the term "Legion" was retained for Roman foot units long after they had ceased to be the fearsome units that Caeser led in Gaul).
As regards to engraved devices on the shields I think that most collectors would not want engraved shield devices since I assume that the shields will be integral with the figure and not separate. It is much easier painting a different pattern than carving the shields off the arms to interchange them with other figures.
Also at this period in history shield devices were fairly simple.
And finally, there are several companies that sell water slide transfers or decals with shield devices in various colours. In this case a flat surface is ideal to apply these. I use this technique with my Greeks and Hat actually sells specific sets of transfers for their Romans although there are other companies like Revo.