External HR sensor recommendation

For just HR the bangle recorder works just fine.

which apps do you use for ECG analysis, Jukka Happonen's?

Yes, I use this for recording ECG on the go.
Locally I use a python app that gets all the data streams on a desktop computer, shows stores and analysis it there. The communication part looks pretty simple "just" initializing and receiving Bluetooth characteristics. The basis was from a blog post example. That link doesn't exist anymore though - just checked. There were a number of options at the time, so I think you should find several examples in the net somewhere.

To me that looks all pretty basic BLE stuff (without being an expert) that I have seen similarly in bangle sw. So I guess it would be easy to write an bangle app for ECG and ACC, maybe even stop/start offline recording in the sensor.
For ECG/ACC maybe just expanding BLE Heart Rate App a bit with additional characteristics.

Is Polar's SDK open source?

Yes there is a open SDK for Android. Its not complete though. All basic stuff is in there, including downloading the internal memory (H10 can record HR without device, not ECG though).
https://github.com/polarofficial/polar-ble-sdk

Could EGC and the 2 Bluetooth devices support be implemented with open source?

ECG yes.
When I checked last I found no way to activate the 2 connection feature open source, but that may have changed (approx 2 years ago). As far as I understand it can be used open source once activated with proprietary app.

Any source of documentation would be appreciated.

Just google a bit. As said my original source is no longer online, but I could find a number of projects on github immediately, including swift or ROS.
Eg:
https://github.com/kylehovey/polar-h10-ecg
https://github.com/wideopensource/polarpy