1. I can't specify, but it's worthy of note that most baroque fiddles still in use have at some point been modified to cope with the higher string tension required by modern pitch. The question really relates to regular alterations in relative pitch for scordatura tunings; any experienced fiddle player already knows that some instruments will accept regular re-tuning while others won't.

2. The construction of guitars is very different, and no useful analogy can really be made.