Hi Vicky,
All information will be kept intact when merging, include campaign history. To read more about what happens when leads are merged, or contacts are merged, or lead to contact are merged, check out
this help article.
In Scenario 1, if the two Contacts don't share an Opportunity, there will be 3 Opportunities after merge.
In Scenario 2, when you merge in Marketo you'll be able to select which fields you'd like to keep. Make sure you select the Master Record first, however. The above help article will go into that in more detail.