Yep would recommend the denoiser - unless you run til noise is down to about 2%, denoising is a good idea (and sometimes even then).
Doing a "generate the UHD Cache" using Load and Append as mentioned in the article, rather than use just one saved UHD Cache from the first camera position, may be the best option. Still shouldn't look THAT different though, and certainly not like an extra light - feel free to share a comparison frame that shows the differences, or raise a bug report via forum or ticket, if you think something is not working as it should.
The 4K Cache is still experimental in the RCs at the moment, so, it may be worth a try, but definitely keep a version of the scene NOT saved from Corona 6 (as once you save it in Corona 6, it is not guaranteed to work in Corona 5 and earlier). Do treat it as an experiment, and take all necessary precautions!
If you use Path Tracing, you may (or may not) get better results, but it may take more time to render (in this case, the lighting in question is on the underside of the balcony, so is almost certainly bounced light of some sort, which is where the Caches come in). May still be worth trying though, as avoiding any flicker can be more important than overall render time.
Let us know how you get on!