I'm not sure what to advise you re opening the envelope but I too had a babybond scan and had them write down gender and put some potty shots in an envelope and i hardly got back to the car before I had opened it. For 2 weeks I was devastated after finding out it was a little boy (no.4) but it did start to ease and I was pleased i knew and that I didn't keep having false hope. For me I think the false hope is worse. I am pg again and I am sure I will end up finding out again cos i know that the 20 weeks or so left of the pregnancy will all but rid me of my GD feelings. This time they are not as strong anyway, as devastatingly I lost my last DS at 24 weeks and it really put gender into perspective for me.
I think I would tell everyone that baby didn't cooperate and you are none the wiser as to gender. If you later decide to open the envelope and you want to tell people you could always say that you organised for another scan which is where you found out. If it is another boy I would just say that that was what you were hoping fo,r as it is so much easier with clothes etc and that you really weren't bothered either way. Although people might doubt whether this is the truth, what can they say, "what a liar??"....I don't think so.
I so hope you get your little and do update us if you decide to open the envelope