1. No we don't currently have any custom field support in the checkout process.
2. No, we don't have support for arbitrary over payment or donations in addition to payment.
3. Yes, you can use PayPal. Payment gateway configuration is part of mojoPortal rather than part of Event Calendar Pro. The same configuration is used by the WebStore feature as is used by Event Calendar Pro. They each have their own checkout pages but rely on components and configuration in mojoPortal. See this article for information about configuring ecommerce:
Hope it helps,