PPL student here with an interest in commercial aviation and procedures, with two probably quite easy questions:

What are the 'small diamonds' shown on a SID/STAR chart, for example the ROSUN1A STAR - they can be seen between LAKEY and ROSUN with the letters SLP next to them (just done a quick search - is it 'Speed Limit Point'?.

Secondly, if an aircraft is being vectored to the ILS, what is the significance of the controller passing the number of track miles to touchdown?

That's right SLP is Speed Limit Point. There can be speed restrictions on a SID chart, but an SLP is more common on a STAR chart as in the example you have given.

The controller will tell you the track miles to touchdown, so that you can plan your descent accordingly. This is particularly important with regard to maintaining low noise continuous descent approaches as well as the general planning.