The best way to keep miles from expiring is probably set an "anniversary" event and buy something inexpensive on that date every year (assuming that your wife hasn't flown or otherwise earned miles in the previous 6 months). That way you don't have to worry about delays in purchases posting to an account leading to headaches getting the miles reinstated.
In your specific case as long as the miles for your purchase eventually post with a date before the 18 months are up you should either keep the miles active or get them reinstated if necessary. If for some reason the date shown when they post is beyond the 18 months, I hope you kept some proof of when you made the purchase.
Jim