Yes, please refer to this page for more information.
No, only the data types specified here can be used and they are all included inside the properties node. Nothing can be defined outside that.
Crawls keep throwing http 429 errors.
Http 429 errors may occasionally happen when doing requests to a Microsoft cloud based service. The publisher component has internal retry logic to try to mitigate the issue but sometimes it might not be enough. In those cases a new crawl can be executed to pick up the items that failed and attempt to index them.