My heart attack moment was during the launch when I started seeing flame flickering around the tops of the engines... and then I thought to look at the altitude indicator, and realised they were at about 10km, and it was just the plume expanding as the atmospheric pressure dropped.

The flames around the top of the engines is normal, it's the exhaust from the turbo pumps feeding the main engines.

Check this page, the Merlin uses an open cycle, where fuel/oxidizer used to fuel the pumps is just dumped overboard.

https://en.wikipedia.org/wiki/Gas-generator_cycle

This picture clearly shows both exhausts: https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#...

Yeah, it's normal, and it usually ends up getting sucked backward, presumably due to the turbulent airflow in the space between the end of the vehicle and the engine nozzles:

Echostar: https://youtu.be/zEfEBJfD_RE?t=18m23s

JCSAT: https://youtu.be/QZTCEO0gvLo?t=17m38s

CRS-9: https://youtu.be/ThIdCuSsJh8?t=17m45s

For some reason that effect was much more pronounced during this flight (at least in the video) than in previous launches that I watched. Maybe it was because of the camera angle looking more at the side of the rocket when usually you are looking more at the bottom.

I got extremely worried when I saw that happen too. I didn't even think to look at the altitude indicator.

I got extremely worried as well and it wouldn't have made a difference looking at the altimeter because I have no idea about how rocket engines behave.

You could see the flicker of the rocket exhaust reflecting on the panel at the back of the drone ship in the split feed so it was at least extremely close when it cut off. Probably the rocket exhaust interfering with the transmission as well as the blast simply pushing the platform out of alignment.

Bit worried myself, One of those grid stabilisers on the first stage was definitely melting on the way back (@26mins on Youtube) just before first stage camera went on the fritz.

It just got covered in soot from the ablative protection (okay, probably some Al2O3 too, from the subsequently burning grid fin) on the grid fins burning off. GTO/GEO missions come in _really_ hot!