How to map optional nested Substitutes data to parent Components using XSLT reliably?

Summary:

I am working on an event-driven integration scenario with a multi-level hierarchical payload. The structure consists of Item Structure(Non-repeating Parent), Components (array), and optional Substitutes (array within Components). My data enrichment process involves fetching missing fields for Components and Substitutes using the Oracle REST API, staging results, and then merging (mapping) enriched data back into the final output payload using XSLT transformations.
The structure is as follows:

• Item Structure (top-level)
  • Components (2nd-level, array)
    • Substitutes (3rd-level, array; optional, may not exist for any/every component)

Processing flow:

• Most required fields for both Components and Substitutes are present in the event payload.