First, there is a misconception that the period in which DVD must decide whether to exercise ROFR is within 30 days of having the contract submitted to it. The actual rule is that the seller must submit the completed contract to DVD "no less than 30 days" before the proposed closing date and then DVD must inform the seller of its decision "prior to the closing date" meaning it could delay decision until the day before closing. In other words DVD must have at least 30 days notice but if it receives the contract more than 30 days before closing it has up until the day before closing to respond. In the typical resale the proposed closing date is usually far more than 30 days after the contract is actually submitted to DVD and DVD usually responds well before the closing date and usually in less than 30 days.
If the terms of the contract are changed such as by the addendum suggested above, DVD could insist on being given at least 30 days after the change to decide and thus if the change occurs less than 30 days before the anticipated closing date it could insist that the closing date be changed to 30 days after notice of the change to the contract. Nevertheless, that does not usually happen and is not likely to happen unless there is significant change to the terms of the contract (e.g., such as a change in the per point price) or the change is submitted just before the closing. The addendum you submitted would likely not affect DVD's decision even if it was submitted less than 30 days before closing and, obviously, if the time to closing was still 30 days or more at the time the addendum was submitted, DVD could not insist on any extensions.