Hi Arun,
In answer to your questions:
1. You can learn about BIAL scripting through the following resources:
Design Studio Application Designer Guide
Getting Started with SAP BusinessObjects Design Studio
2. It is not possible to modify the functionality of the standard Design Studio components via the SDK. You can however modify the appearance of standard components by applying CSS as shown in this tutorial video.
3. You do not need to download the UI5 Development Kit to incorporate UI5 controls into SDK components. To access a UI5 control you need to develop the component using Handler Type "sapui5". The approach is described in Section 6 SDK Extensions Using SAPUI5 Controls of the Developer Guide. You can also download the SDK Templates and Samples which include UI5 examples.
Regards,
Mustafa.