Even with the power of Google, it sometimes helps to have a bit of specialist insight and background into what you're looking for. I didn't find it first search either!
Lovely part of the world - I have fond memories of a mountain marathon I did in the Rannoch Moor area about 30 years ago, and also getting out and about while I was working near Stirling for a short while in the mid 90s, but it's a long time since I've been up there. (Flippin' eck, I'm old!
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Probably. There's a lot of different stuff out there for overhead conductors, but I'm pretty sure it's Rubus conductor - IIRC a nominal 500mm2 All Aluminium Alloy Conductor AAAC. It's not hollow but has a series of counter-wound layers of smaller aluminum strands. Google images may be better than my explanation.
It's not my area of expertise but I understand that all aluminium conductors have become more commonplace because the alloying technology has improved to allow the elimination of traditionally stronger and heavier steel cores that were found in earlier ACSR (Aluminium Conductor Steel Reinforced) conductors. Like you say, weight is a factor when hanging stuff in the air, so is cost (copper is comparatively expensive even though it's a better conductor), so is electrical performance (Al is better conductor than steel) and also thermal performance (when Al/Fe metals expand at different rates in combined conductors as electrical load changes and the conductor changes temperature it can cause functional problems over time, accelerated deterioration or need for more maintenance or replacement).
(And before you start thinking I'm amazing
, I'm not, I can't really tell by just looking at your photos. I might still be wrong but fortunately, this is Beauly-Denny line, a relatively recent upgrade to GB Transmission System, in essence just rebuilt from 132kV to 275kV but it's physically much bigger structures and higher in the air - it's very well known in the industry because of the consenting challenges it had prior to construction. Some facts/info just sticks in the brain!)
Now I really would be guessing - I don't know what the box is for, but you could very well be right that it's a traffic monitor - certainly looks like a very good hypothesis seeing the induction loops I spy in the road behind your bike.