For clients who want to use DX API's in 8.3 version, are there any guidelines to be followed to minimize remediation efforts later (while upgrading to 8.6 - 8.x)?

DX API v2 is only available in 8.5 - If you use Pega 8.1 to 8.4, you can use the DX API v1 - this api will no longer receive major updates - At this point, if you start to use the DX API, we highly recommend to start with the v2 on 8.5

Are the DX APIs v2 extensible? for example, we get metadata with DX APIs, is it possible to enrich the metadata being sent ?

Not right now

Can DX API be used as Pega mashup?

Yes you can implement your own mashup using the DX API

How DXAPI and Cosmos are related?

DX API is back end - it is independent of Cosmos - Cosmos is the user experience that you can use if you don't want to use the DX API directly

How flexible is the DX API V2 services in terms of mapping data to the clipboard? This seemed a limitation of the DX API V1 in terms of mapping data to one page.

in dx v1 api you can use embedded pages and pagelist for tables./ rdl - I would make sure that you are running the latest version

Now that the UI is rendered in react and using DX API...would the Pega apps be stateless and don’t need sticky session anymore?

Correct- the DX API is stateless and does not require a sticky session