I guess I had remembered my message below as having been a bit more specific than it was. So, here is specific wording for the second part of Section 7.1. In the general case, such declarations will not be available to or used by the signature verifier, particularly for non-signature XML, which may be in othr namespaces, in the same document as the signature.
Attributes having default values are explicitly present.
Attribute value white space is normalized. In generaly, it seems like mandating these syntax contraints would be a good idea. I guess that's right, ie, characters not representable should be changed into a numeric character reference. But I can't see how it matters whether it is decimal or hex. Any conformant XML 1.0 reader will translate the numeric character references into the appropriate character.