The symbol on a yellow rectangle is, as someone else said, is a diversion. Symbols are used to differentiate between multiple diversion routes. If there’s no text, you’ve missed the start of the diversion (ie, it doesn’t affect you).
The ‘faded clearway’ sign sounds like a faded clearway sign. I can’t think of anything else it can be.
The roadworks speed limit signs -should- contain a regular speed sign inside the yellow box, as per the (50) sign pictured above. It’s possibly an invalid sign without this; I suspect that it’s intended to be mandatory but may be an exploitable loophole if you were to get caught speeding. How brave are you feeling?
Related to this, matrix signs showing yellow flashing lights next to speed limits are advisory only. Mandatory instructions are accompanied by red flashing lights. Some of the new ‘variable speed limit’ road sections have matrix displays which show the red circle as per a regular sign; these too are mandatory.
The Exclamation warning triangle means “here’s a hazard we don’t have a specific sign for” – it should be accompanied by a plate underneath elaborating on what this is. Perhaps it’s fallen off. Here’s an example,
The NSL / (50) on the same post is presumably a mistake, assuming one or the other hasn’t been turned round 180′ or something. Is there one on the other side of the road that might give you a clue? At face value you’ve got a sign saying ‘end of restriction’ and another saying ‘new restriction’ so logically the 50 would take precedence. I reckon they’d have a hard time prosecuting you for an infraction though, assuming you contested a ticket.