One way to look at it is that people who donate earlier receive a bonus as compensation for the additional risk of not knowing what their final share of the distribution will be. The longer you wait, the more idea you will have of your final share, but the lower the bonus. Waiting until the very end gives you the most definite estimate of actual share, but this period also tends to see a large spike of contributions so it can still be pretty chaotic. Waiting until the very last minute gives you the most precise view of expected distribution in return for contribution, but also runs the risk of not having the donation transaction processed in time to enter the blockchain.

There were a few disputes in the Mastercoin ICO over transactions which were submitted to the blockchain prior to the deadline but didn't confirm until after. Fortunately, Lisk has a 10 second block time which makes this less of an issue on its end, but Bitcoin has a highly variable block time averaging 10-20 minutes so donors might want to account for this if they wait until the end of the ICO to donate. My preference is that eligibility of donations for distribution of Lisk should be time-stamped when they first enter the Bitcoin blockchain, not when they are confirmed, but with an intermediary platform like Shapeshift this could be more complicated.

There should be clear definition in the ICO terms which timestamp(s) will be used in determining eligibility for Lisk distribution. My preference is time of submission, when the transaction first enters the blockchain.

XCR does not receive a bonus for earlier conversion. Since XCR already represents an existing share in the distribution regardless of outcome regardless of when it is converted, it makes sense that it doesn't receive a bonus.