How to define and manage new Custom Object and it's association with Lead/Campaign/Program?

I would like to understand more on Custom Object management and usage.

## What are the steps to define a NEW Custom Object?
1) Manually by user
2) Programmatically (via API) - this is preferred but could NOT find an API for the same
 Kindly illustrate or provide document references.

## How can I associate a Lead and Program/Campaign with a specific Custom object record.  For example, can we design Custom Object as below - where a lead is participating in many programs/campaigns and has specific info for each record?
  - leadId (maps to ID of lead)
  - programId or campaignId (maps to ID of lead)
  - webinarID
  - webinarSubject
  - dateTime
  - ...
  - ...

Here, the Custom Object holds data for lead from each campaign/program participated in specific webinar.

## As per above design,
  - Does Marketo validate mapping between Lead (id field) and Custom Object (leadId field) ?
  - Does Marketo validate mapping between Campaign/Program (id field) and Custom Object (campaignId/programId field) ?
   - Custom Object > Can we define a combination of leadId & campaign/programId as unique constraint/key
  

## Using Email Scripting
  - An Email script can be referred from Email Template. At runtime, what is the scope in which the Email script runs?
   - Can Email script look up a specific (unique) Custom Object record (as per above data structure)? If yes, then kindly illustrate.

 Thanks in advance.