Using Disney gift cards or any other form of payment for any part will also negate the OBC offer.
except for the disney rewards card, where you load the points from your visa. if you use that to pay a portion of the cruise, they consider it equivalent to using your disney visa, and you'll still get the $50 drink credit. i've done it twice, in feb 2016 and oct 2015. last time i specifically asked about it and was told they consider the rewards card just like using your visa, since the rewards come from visa spending.
as for other questions:
you're supposed to ask for the credit when you make final payment, but often they will add it later - just call and ask.
to get the credit, you have to make all payments with the disney visa, or rewards card. the 6 months interest-free will apply to any
disney cruise payments, so if you split the cruise between your disney visa and another card, you will still get the 6 months interest-free on the portion you paid with the disney visa. however, you would not get the drink credit if you split the cost of the cruise between disney and non-disney cards.
if you pay your card off every month, be aware that after 4 months disney will apply your payments to the cruise first, so you will end up paying interest on the new charges. so basically, the 6 months without interest is really 3 or 4 months. this is what i've been told on the phone by disney reps three different times. because of this, i've always paid off my cruises in 3 months, just to avoid any hassle. so, i don't really have any experience with whether it's true or not. others have posted on here that's not happened to them. but, as i've been told it multiple times by different people, just wanted to mention it.
the drink credit is not combinable with the credit for on-board bookings. it usually is combinable with any credit your TA may be offering, though.
fwiw, on my feb 2016 wonder cruise i got a note onboard saying my $50 visa credit was only good for drinks, but in oct 2015 on the magic i had a note saying the credit was good toward anything. i did spend over $50 at the bars so i'm not sure where the credit was actually used, though. : )