But there are’t three different situations. there are at most two. To clarify:
we have scenario 1a which is a constructed minimal reproduction of the primary reported bug/feature request, namely that the emoji parser is not working inside of [spoiler] tags
[spoiler]:+1:[/spoiler]
scenario 1b is a reproduction of the discovering scenario, where the emoji was embedded in a larger block of text
[spoiler]prefix text :+1: Suffix text[/spoiler]
These two scenarios are the same issue: namely that the :+1: emoji code was not parsed when inside [spoiler] tags.
when constructing the expected results for the bug report another issue, that is likely related to the parsing code that determines spoilers, was discovered. This is arguably an different bug but is noted here because it causes the expected results of the second case to display incorrectly.
This Scenario 1b expected results was created by pasting the HTML for the 
emoji from the emoji_one emoji set in place of the :+1; emoji code in the spoiler.
[spoiler]prefix text <img src="/images/emoji/emoji_one/+1.png?v=0" title=":+1:" width="20" height="20" class="emoji" alt="+1"> Suffix text[/spoiler]
a second example of the separate but likely related issue was generated in my first reply to you:
[spoiler]prefix text <img src="//cdck-file-uploads-global.s3.dualstack.us-west-2.amazonaws.com/meta/original/3X/2/4/245f54ed160ca0a7315e55e66f50ffda001ba9f9.png" title="discourse"> Suffix text[/spoiler]