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I don't do orchestral tracks but every underscore version I have provided to a library has lined up with the full track version start to finish.

Long silent parts can be awkward - usually I try to leave something in when that happens even if it is only a pulse of some thing subtle

Probably best to ask the library just in case.

Ask the library if they want the ALTs the same length as the Original
One library I'm working with doesn't care about matching the length for ALTs
They understand the importance of the musical piece working as a whole