In short, no, the nikah will not be invalidated by such a condition. At the time of marriage, when conditions are stipulated the spouses are required to fulfil them just as they would uphold a promise that they made, but failure to do this does not invalidate the marriage. Moreover, if the conditions are contrary to Islamic teachings they are void anyway and have no bearing whatsoever and nor is either party required to fulfil them. The other issue, is to give the wife the right to divorce her husband. Although permissible, this should be abstained from as much as possible. There is a reason Allah has given the right to divorce to the husband and one should be mindful of the wisdom behind it. This right should only be excercised in certain extreme conditions. With regards to husband marrying again, its not always as intolerable as it initially appears to be. A woman should be careful about preventing her husband from marrying again as under certain circumstances, it is more beneficial for her if he does marry again than for her to restrict him to her only when he clearly is in need of a second wife. If he does manage it, it will not be without resentment and she will be making unlawful that which Allah has made permissible. My advice is to not worry about stipulating any conditions in marriage except in abnoramal situations. What seems extremely important before marriage is often not as significant after marriage as with marriage, lives, personalities and perspectives often change dramatically especially when the spouses marry at a young age.