The Sign of Three -- spoilers below the cut.
About the wound:
No, Sherlock DID know where the wound was. But he described it poorly during his speech.
"In the stomach" makes most of us think "wound in front of body" and not in say, the kidneys. The wound is in the back, as shown in the ep, not the front. This is actually right where it should be; remember it shows where the murderer is each time with the victims.
This particular murder method did happen historically and in the same manner. As murders go, it's pretty handy because it makes the victim think s/he just had a sudden pain, if it's noticed at all.
We all get sudden sharp pains. A stitch in your side, etc. You don't automatically think, "Oh God, I've been stabbed!" And when Sherlock reconstructs the crime, he shows where the murderer stabbed his victim each time -- in the back.
But the point of Sherlock's story was to show how great John was. While Sherlock was trying to solve the puzzle-crime, John was rescuing the victim.
Multiple computers... IP addresses!
You can use different browser window tabs, sure. But they'd all have the same IP address. That's how your computer is ID'd to other machines.
Each machine should have a different address. The IP gateway for all the machines in 221B would be the same, but that's simple enough. The actual building uses one gateway.
I don't know if SH suspected he might be watched on the ghost forum. It was a smart move. That way if any of his chat companions did happen to be a geek, one who could "see" where his computer was, they wouldn't realize every "Sherlock" was the one Sherlock. Not unless said user could backtrack along the IP addresses to see their common gateway...
I bet, although we didn't see that, that each "Sherlock" ID was "Sherlock01" etc. 8) It might have been like Twitter, even, where you can have a user name that changes [SherlockLives or S Holmes] whether or not your ID name changes [GeniusHolmes].
And last... I think Sherlock was just -- just! o.o -- panicking. I totally understand. I have some social phobias myself, along with being HSP (Highly Sensitive Person).
I also agree that our Sherlock is very probably a HSP. Really glad I refound that link, I lost track of it in 2012.
Bonus: Sherlock does not have Asperger's or Autism, Thanks - From 4 Pyschiatrists. And if you haven't read wellingtongoose's other essays, you should, she's lovely!