So for Attending the meeting Do we need to use the same SDK access & secret keys for my 100 participants(join only) to initialize the SDK or I need to have separate SDKs ?
Your SDK key/secret are only use to initialize the SDK. They have absolutely no effect on which account is being used to join or start a meeting.
I can call Zoom client app but the meeting details are visible for me, I am not able to control the options if I am not opening the meeting in my application.
Apologies, but what exactly are you trying to accomplish here?
As a side note, the plan type you have will not impact whether or not it is possible to use the SDK. If you use valid SDK credentials to initialize the SDK, an end user could join a meeting without having a Zoom account at all, similarly to how the Zoom client works.