Is forty-eight hours too short notice to invite people?
It is short, but not rude, and saying so plainly is the politeness. Most people can move a Friday night for a day's notice. Two hours asks something bigger, and that kind of message works best inside a close group that already expects you to text.
Should a last-minute invite go to the group or one by one?
Group is fine and usually faster, with one caveat: the first yeses matter most, so send the group message first, then ping personally the few whose presence actually decides the night.
What if almost nobody replies in time?
Then the party shrinks honestly. Send the reminder once, plan for who answered, and eat well anyway. A last-minute table of six people who wanted to be there beats forty maybes who needed the space.
Do I still need a reply link if everyone has their phones?
Especially then. At short notice everyone's phone is already open, which makes one tap faster than typing into a group where three other conversations are running. The list builds itself while you are doing everything else.
How late can the reply deadline be?
Tight enough to force a real answer, generous enough to be believable. For a tomorrow-night thing, noon works; for Friday, Thursday evening. A deadline that passes while everyone is at work means the reminder reaches the unread, and nobody is served by that.