"...will UK/EU passports be legal travel documents for Europe."
There is no such thing as a "UK/EU" passport. There is a UK passport, and that's it. UK passports are valid in all countries in the world.
"What an EU passport for a non-EU national will command in terms of consideration is rather less certain and in the end some governments, particularly outside Europe, may decide not to accept them because they will then be known to (strictly) be a misrepresentation."
Once again, there is no such thing as an EU passport. The EU does not (yet) issue passports. The passports issued by the UK make no reference to the EU at all other than to have (for a reason unknown) the words "European Union" embossed on its cover. After 2019 those words will presumably be removed from new issues. The old ones will remain valid and be treated exactly as United Kingdom passports are now. As I said in response to another question, border authorities outside the EU are not interested in the European Union for passport purposes. It is individual EU member nations that issue passports and it is that which bestows on them their validity. There is not a chance in hell of a UK passport being viewed any differently after Brexit.