The first question is one on which the IRS is not particularly helpful in its guidance to the public.  You can obviously hold the event. The question is whether the value of the event will be deducted from the amount that the donor may claim as a charitable contribution deduction.

In Publication 526 on charitable contributions, the IRS talks about deducting contributions from which the donor does not personally benefit and not deducting gifts from which the donor does benefit. It says that where there is a benefit to the donor, the donor can deduct only the amount that exceeds the value of the goods or services received “as a result of” the gift.  That would seem to suggest that if you gave previous donors a benefit because of their prior gift, the deduction amount would be reduced by the value of the benefit.

The Tax Regulation, however (Reg. 1.170-1(h)(2), says the reduction applies only where the gift was given partly “in consideration for” the benefit.  The Reg further defines “consideration” as being received “if, at the time the taxpayer makes the payment, … the taxpayer receives or expects to receive goods or services … in return.”  In your case, since this event is a new idea, not advertised to the donors in advance and not expected by them, their gifts would not be “in consideration of” the benefits and they would be entitled to the full deduction.  (See Ready Reference Page: “Charities Must Set Value on ‘Quid Pro Quo’ Gifts”)

Your second question is an entirely separate issue. When a donor must reduce the amount of a contribution deduction because the donor receives goods or services in consideration for the gift, the reduction is based on the value of the goods or services received (and not the cost to the charity of providing the benefits).  It would make no difference to the donor whether the charity paid for the benefit or had a sponsor to cover the cost.  Therefore, without regard to whether the original payment is a quid pro quo gift, it seems pretty clear that the charity will benefit if it finds someone else to cover the cost of the event.