Will my MacBook Air 6,1 support MST (Multi-Stream Transport)?
Your MacBook Air's hardware: Yes.
MacOS (as tested with macOS Sierra 10.12.6): No.

I've tested it myself with an older MST capable Dell monitor and found various forum entries about this topic. No chance with macOS.
The Intel HD5000 graphics chip on your Macbook is capable to do MST with DP 1.2.

MacOS does not have any MST support built in. There is a 1% chance it will work with High Sierra 10.13 (if somebody can test this?!) but I really doubt it because Apple does not care about daisy chaining with MST when they have their Thunderbolt ports (which require a Thunderbolt monitor).

Only way to get MST working may be using a native Windows installation with Bootcamp on your Macbook.