The problem with asking for RAI is that unless you have a specific statement from the designer of the game as to their intent, you are asking for pure supposition, which is largely then substituted with the answerer's opinion. We have a previous meta question on the nature of and problems with RAI in a question. Rules-as-intended and designer intent You specifically use the term incorrectly as "inferring" intent. As you note, we don't have Detect Thoughts, and people arguing their own inference of intent litters RPG forums everywhere. We prefer answers that Back It Up! here.
If you want an answer from pure RAW and strict logical implications of RAW, you ask for RAW. You can modify your list of sources to include "Tweets by Jeremy Crawford" if you want.
If you want an answer that takes into account not just the rules in the book, but common usage, play experience, guidance from other editions, etc. you just ask the question.
Your question about "can I write spells into a normal book" is a great example. The RAW answer apparently is "yes but you can't do anything with it..." The general answer is no, no one plays that way, a spellbook is the special kind of book you put spells in and any other "book" is not suitable. The only way to have a RAI answer is if someone asks that specific question to Crawford and gets an answer back. The degree to which people don't understand that leads us to prefer you to just say "RAW including designer tweets" or nothing, not use the term RAI.