Convert JSON to XML online without writing tags

Each element of the array is written as a repeated element with the same tag name, which is the normal XML idiom. An array of three features becomes three feature elements rather than one containing an index.

Neither is a valid XML element name, so it becomes an item element with the original key preserved in a name attribute. The data survives and the XML still parses.

Yes. Ampersands, angle brackets and quotes in values are escaped to entities, so content containing HTML does not break the document.

As an empty element carrying the xsi:nil attribute, which is the convention XML Schema defines for a null value.

Yes, with the XML to JSON converter, though a round trip is not always exact. XML draws no distinction between a single element and an array of one, so a one item array comes back as a single value.