a flight booking plan - is there a downside to this?

I need a one way long haul flight back to edinburgh in November

all one ways, as u know, are turbo £

if I book a return and just don't use the return leg, how likely is the airline to come and sue me?

You'll be fine, I can't see how/why they'd sue you

I dimly recall seeing a story about an airline (Lufthansa maybe?) going after a customer for the additional price of the one way ticket

Make something up.  I couldn't fly because I was flatulent / hungover / had a cold.  Because I would have been denied boarding due to a typical night out is a plausible one.

Like the alarmingly regular p1shed pilot occurrences at Glasgow Airport.

You’re fine if it’s the return leg you’re skipping. If it’s the outbound leg you skip, then they automatically cancel the return flight.