- The first sentence is easier to follow if 'dankbar' is moved -> ich bin Ihnen in aller Ewigkeit dankbar für Ihre Hilfe...
- The same has happened in the second sentence. You were going to put 'wünschen' at the end of the sentence but because it was so far from the beginning of the sentence, you ended up forgetting all about it. How about
Ich möchte Ihnen und Ihrer ganzen Familie ein wunderbares Weihnachtsfest wünschen ?
- eternamente in Portuguese has another connotation than 'in aller Ewigkeit' in German. It sounds exaggerated in German (much less so in Portuguese). Better, in my opinion, would be the simple 'immer'.