1) That's pretty much what I'd recommend (my moral stances aside.) I'd suggest they be willing and prepared to let each other know and to respect it if one or the other decides they don't care for this arrangement and wants to return to monogamy. And above all, take care that you don't experiment in dangerous settings, or pick up anything you wouldn't like to share. Condoms may be less fun, but they are pretty much essential.
2) Track her down - the college is a good place to start. You can hunt people down through the internet for a price, as well. And no, you can't take it out of what you owe her. If you're reluctant to confess, send a money order signed "Jane Smith."
3) Excuses are like ... Well, never mind, but everybody has one. You have several, as did the previous letter writer. Your marriage has already lasted more than many. Discuss this with him. Tell him a promise made so long ago, especially under the influence, can be a hard thing to keep now. Try to negotiate a bit. Perhaps you don't need to be there the entire five years. Perhaps you could live where he already has contacts. Perhaps your job has contacts or an office in Germany you can transfer to, though not necessarily in Berlin. Just because you agreed to live in Germany for five years doesn't mean the entire five years necessarily must be on his terms only.
Before bearing children out of the country, find out what the law is regarding their citizenship in both countries. Last I heard, Americans born outside the US must live in the US a certain number of years to retain citizenship. Some countries require people born there to report for military duty at a certain age, wherever they live or if they return for a visit. Maybe you can spend extended vacations in the US when that time comes, and return to Germany with the child(ren) in tow. Also, in case of divorce, whatever country the kids are in at the time usually keeps them. Make sure you have legal paperwork dictating what happens to the kids if you both die, once you have any, or there will be an international incident with both sets of relatives wanting custody.
4) Didn't you learn how to deal with this in kindergarten? If you ignore the bullies who tease you, they go on to a target who is more fun to tease. Ignore their e-mails. Reply with a link to Snopes if you like - I do for the more outrageous ones, and ignore any responses. I hope you aren't in an executive position, if you can't deal with something so simple.