Preferably, send the gift to the bride before the wedding or to the couple soon thereafter. In some regions gifts are brought to the reception and placed on a special table. Contrary to a current rumor that you have a year to send a gift, it really should be sent right away or within three months of the wedding.
No. A registry is for your convenience and you are not limited to what is on the list.
There is no rule, so it is entirely up to you. Let your affection for the bride and groom and your budget be your guide.
In some cultures it is THE traditional gift. If you are uncomfortable about giving cash or a check, you can give a gift certificate to a store where the bride and groom are registered.
Ouch!! This is an awkward situation. It is certainly okay to call and ask the couple if they received the gift. If you find this too awkward and the gift was sent from a store, you can call the store and have it traced. Just as a gift should be sent right away or within three months of a wedding, a thank you note should be written right away or, at the very least, within three months of receiving a gift.