On your first question about the picture description. The identifier enclosed in TN indicators is used ONLY when there is a caption. The caption is actually in print, so it has no TN indicators. The identifier, Picture, is enclosed in TN indicators because that word is NOT in print and it is placed there by the transcriber to inform the reader what type of illustration the caption refers to. Look at the example 6-3 that follows 6.3.1. Pretend that the picture there did NOT have a caption. Notice that the entire description is enclosed in TN indicators without an identifier. If you want to identify what you are describer, include the identifier in the description.
TNPicture shows girl holding a red balloon.TN
Look carefully at the surrounding text. If there is relevant material in the text, use it to determine the description. If the text mentions something like, The picture tells the whole story about the girl, you can simply describe the picture TNThe girl is holding a red balloon.TN If text does not provide a description or part of a description, then you describe it more completely. Use judgment.
For your second question, the M is not a single letter all by itself. It is part of a sentence and the reason it gets a letter sign is so that it won't be read as "More." Pretend the M is the first word of the sentence, which is really what it is here.
Double italics letter sign cap M is for close italics music.