No, I'm not a mechanic, but ...
When getting my last two cars serviced - both Toyotas - I used to get the offer for a fuel injector or other form of engine clean every time. I wonder if Toyota Australia (including Lexus) service consultants are trained ask if you want fries with that - or, should I say, "You NEED fries with that!"
Since buying my second Toyota, I resolved never to accept the offer to clean such build-up; instead, if I continue using premium fuel with added cleaners, that should be enough. According to Caltex, you just need two consecutive full tanks of Vortex to significantly clean your engine. Seeing that I almost always used 95-98 RON fuel on my engine (even though it could take 91 RON), I got good power and a clean engine to boot, without needing cleaners or additives at servicing - at least, that's the theory ...
Nowadays, all premium fuels sold in Australia contain cleaners (as well as Shell 91 Unleaded). Question is, have you always been using premium unleaded in your IS, as recommended? If so, then either the Lexus service rep is lying, or my theory doesn't (always) work.
As for the aircon re-gassing - not sure there. I've never had problems with Denso aircon over 13 years.
Cheers