Answer 1

It's possible your Android Chromium Webview is locked to your OS version, making it an OS limitation (vs. the app).
Chromium Webview and Chrome tend to update simultaneously, so you can usually just check what Chome version added the feature:
https://caniuse.com/#feat=css-scroll-behavior
It was available only as a browser flag* starting with Chrome 49, fully enabled at version 65.
You can check what version webview you have with the HTML5 tests page, included here as an iframe:
https://code.sololearn.com/WeBKGPjY0UHK/?ref=app
* I believe flags are not accessible from the webview component; it was done this way by browser vendors because browser prefixes have been greatly abused.

Answer 2

This may help:
https://css-tricks.com/almanac/properties/s/scroll-behavior/
...especially the bit about 'fancy javascript':
https://css-tricks.com/snippets/jquery/smooth-scrolling/