Answering your queries in this thread:
Yes. You can visit GitHub to check Quickstart guide and detailed documentation here. You can use community for discussing all things related to API.
No. Currently we do not allow paper trading, as it still lies unclear in terms of regulations.
On stability and response times for APIs, you can find a lot of threads on this community also where users have even stress tested APIs and benchmarked the same. Referencing one by @Gangavarapu here - Dhan API Stress Test- Dark Magic? - #8 by Gangavarapu