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How does these abilities interact?

• Fast Recovery + Infernal Health:
  Is the effective Hit Point multiplier four when both of these apply?
  And do they apply in similar ways to the same types of rest? They have similar wordings, but Infernal Health is more specific.
  
• Fast Recovery + Hillock Halfling:
  Do you double the extra Hit Points gained by Hillock Halfling?
  
• Fast Recovery + Long-Term Rest:
  Do you heal by twice your Constitution modifier multiplied by twice your level for a total of 4 * Con * level?
  
• Fast Recovery + Verdant Metamorphosis:
  Does a 10 minutes rest heal you to maximum Hit Points?
  
• Solar Rejuvenation/Vivacious Conduit + Fast Recovery/Infernal Health:
  Are the Hit Points gained by resting 10 minutes from Solar Rejuvenation or Vivacious Conduit affected by Fast Recovery or Infernal Health?
  
• Hillock Halfling + Long-Term Rest:
  Does Long-Term Rest benefit from the Hillock Halfling heritage, and if yes by how much?
  

The interesting parts of each ability are:

Fast Recovery

You regain twice as many Hit Points from resting.

You regain triple the normal number of Hit Points when resting (meaning you regain triple your Constitution modifier multiplied by your level). The healing you gain from long-term rest is similarly tripled.

When you regain Hit Points overnight, add your level to the Hit Points regained.

You can spend an entire day and night resting during downtime to recover Hit Points equal to your Constitution modifier (minimum 1) multiplied by twice your level.

If you rest for 10 minutes while transformed into a non-creature plant during daylight hours under direct sunlight, you recover half your maximum Hit Points.

If you rest outdoors for 10 minutes during the day, you regain Hit Points equal to your Constitution modifier × half your level.

If you rest for 10 minutes, you gain Hit Points equal to your Constitution modifier × half your level.