Who is on the line when parents are hosting?
Traditionally the hosts ask, so the parents' names head the invitation and the couple appear in the request. When hosting is shared or the couple is paying, the header can name both families or simply the couple. Whatever you pick, the header should match who is actually doing the hosting.
Do I put a phone number, a card, or a link for RSVPs?
A link, with a phone number as the fallback for older guests. A reply card works if you will truly collect and count them; a page records each answer against a name automatically, including plus-ones and meal notes, which a card cannot do.
Where does the dress code belong — card or page?
On the page. A single word like 'black tie' is safe to print, but the useful version of the dress note explains the ground, the weather, and the shoes, and all of that changes. Put one short hint on the card and the full sentence on the invitation site.
How early should the invitations reach guests?
Eight to twelve weeks before the day for most weddings, with the reply deadline three to four weeks out. Send save-the-dates earlier for anything guests must travel or book around, and keep the wording on those cards short.
Is it acceptable to skip the formal request line entirely?
Perfectly. Plenty of couples open with 'We're getting married' and a date, and no guest is offended. What matters is consistency: the awkward version is a formal header that drifts into casual wording by the dinner note.