Applying For A New Visa When Old Visa Is Still Usable

OK, the title may sound strange but here's the situation:

My friend has been to the US before on another visa and currently has a 2 entry visitor visa to the US. The first entry will be used as normal. The second entry would be used several months afterward. The problem is there has been a change in our itinerary. One possibility is for us to enter the US and then go on a cruise which would be stopping several places in Mexico during our visit. Now obviously we would need more entries to go on the cruise.

1) Most importantly, if we tried to reapply for a visa in between visits, and happened to be rejected, could they (or would they) invalidate the current unexpired visa?

2) Would it look bad for future visa tries if we try to reapply (in an attempt at getting more entries)?

3) While this is many months away, the planning needs to be done now. Is there any hope, if we reapply now (without using the visa at all), that the immigration official might change the visa if I bring all the emails and other documents related to the itinerary change? The problem is I obviously wouldn't book a cruise and then hope for enough entries on a visa. I need to be sure ahead of time.

Any advice appreciated. The most important thing is that we don't want to lose the visas that have already been granted. Perhaps I should stop dreaming and just scrap these plans altogether.