Ja, das funktioniert für jede Relation, deren ID man kennt. Einfach 3.600.000.000 zur ID (beim Harz zu 3734750) addieren. In Deinem Fall also 3.603.734.750 eintragen (natürlich ohne die Tausenderpunkte).

Yes, JOSM is able to display the length of selected ways. Just use the selection tool (Key “S”) and select the way. In the status line of JOSM you see a little ruler which displays the total length of all selected ways.

If the way consists of more than one part just multi select all ways using the ctrl-Key and mouse click. The length will be summed up.

Best regards a_peter