Pre Order

It’s possible to complete this action from Magento product grid provided you have an option to sort products by category (this feature is missing in Magento out-of-the-box but can be added through customizations or extensions like Extended Product Grid).

In order to set products from a particular category to Pre order, you need to apply a mass action called “Update Attributes”.  On the inventory tab you will need to choose “Allow Pre-Orders” in the backorders field:

Yes, there is a special column where you can filter pre-orders:

No, you can't split an order as the possibility is currently not supported by our Pre Order extension.

The product’s Availability is Out of Stock.

Solution:

1. Turn off the option that automatically enables the Out of Stock status for products that reach a particular quantity limit.
2.  One of possible solutions is to reconfigure preorder settings. For instance, you can enable the preorder option when the product’s quantity reaches a particular limit but still is On Stock.