Thank you for your questions and for the examples.
To your first question: When a key is created in Nemeth, the whole tactile should be encapsulated in Nemeth. This is in accordance 1) with §5.7.2 of Guidelines and Standards for Tactile Graphics 2010, which says, "The texture, symbol, or key used in the graphic must be an exact match to that used in the key listing." and 2) with a preference to reduce the number of switch indicators used in a transcription (i.e., better to have the whole graphic in Nemeth than to enclose individual labels within switch indicators within the graphic.)
So, I believe your Example 2 is technically correct.
To your second question: To the best of my knowledge, within Nemeth Code a numeric key can be used to represent items in a tactile graphic even when numbers that are not part of the numeric key are also used in that tactile graphic. See §187 of the Nemeth Code, which does not prohibit the use of a numeric key in the presence of other numbers. Nonetheless, I would consider using an alphabetic key instead of a numeric key in your tangram examples. I hope that using a key like sv, fi, tf, and wf instead of ⠼⠁, ⠼⠃, ⠼⠉, and ⠼⠙ would help the reader avoid mistaking a numeric key value for a measurement value.
Either way, I applaud your question and your approach to keying; they are thoughtful without being overdone.