Snapedom

The moment Draco gave up on the attempt to kill Albus the third clause of the Unbreakable Vow kicked in (which is why it was impossible to expel or suspend Draco that year), so Severus had to do it ASAP regardless of circumstances and audience. If anything it was Albus who forced Harry to be the witness. Albus knew before he left the school that Draco was going to make his move that night and he knowingly took Harry with him.

As for delivering the message - the trigger to deliver it was that Nagini be protected. The first time Severus saw Nagini under protection was already in the Shrieking Shack. We do not know if Severus made alternative arrangement - in any case his preference was to do it in person. But the message would have been delivered regardless of whether Severus prepared an alternate route of his own - via Albus' portrait and the other castle portraits if not by any other means. (Albus could move about the castle portrait frames until he could contact a ghost or a house-elf and tell them to tell Harry to meet him somewhere.) The only problem with that method would be that Albus would have had to face Harry before Harry could know he was going to survive, and that's a huge inconvenience to the Twinkly One. As headmaster Severus knew what the portraits were capable of, so he may have figured it out himself. But he preferred the direct and personal route because it allowed him to apologize to Harry via his memories.

(BTW summeriris, you may be interested in my latest attempts to untangle the mess that was Albus' endgame plan here).