Whether or not he has forfeited immunity is a matter for a tribunal to decide and has no set in stone answer. Magi can forfeit immunity "while committing, preparing to commit, or shortly after committing a crime. By his criminal act his legal protection is forfeit. While forfeit other magi may act against him, but the response should be proportional".
Is a year and a half "shortly after"? Is he preparing to commit another crime? If so immunity is forfeited but in the end it would have to be decided at tribunal.
Scrying to find his identity is probably a better idea than very publicly killing an unknown magus (and someone probably will detect your sigil and find out it was you). Better yet, get a quesitor involved - they have some leeway to legally scry on magi while investigating crimes.
I don't think forfeit immunity would be a consideration after that much time has elapsed.
That said, if nobody seems to recognize this mystery magus, there's a decent chance nobody will miss him enough to bring charges against you at tribunal if you do decide to commit an Hermetic Crime. The most sensible thing to do would be to use Intellego Vim to discern the perpetrator's sigil and either turn it over to a Quaesitor or try to investigate the matter for yourself. Unfortunately, you probably missed that opportunity...
Yeah, I think you had your window to kill this chump when he first attacked you, but now that he's gone, it's a matter for the Quaesitores. But if you have an Arcane Connection, that's a great piece of leverage. Fix it. Do not use it to scry on him! This would open you up to charges of breaking the Code yourself. But prepare for him to attack you again, and perhaps set up an InVi on yourself to know when someone is scrying on you. Eventually he will show himself again, or get caught by Quaesitores, who will then try to negotiate a settlement. You can make it clear you have an Arcane Connection, allowing you to roast him, and that should convince him to let it go. If he's smart.
He attacked you while he was committing a crime, which is a double violation of the code. I'd say fix the arcane connection, try and discern his identity in a mundane fashion, and turn him over for prosecution at tribunal for his violations. When to involve the quaesitores is very much dependent on what tribunal you are in. Do you have arcane connections to anyone who was associated with him during the crime? Divining their location would certainly not be a violation of the oath.
Yes, it is neither scrying nor depriving them of their magical ability, though it makes the process of gathering evidence difficult.
For that matter you could teleport the magus to you with the arcane connection then challenge them to certamen whereby they give you their name and sigil if they lose and you give them the AC if they win, and this too would be perfectly legitimate.
If you've got that AC, you could use the InCo spell from HoH:TL to figure out who the person is. Yes, you'd be scrying.
Teleporting the magus isn't easy. AC range with an AC length teleport is a Ritual, and you'd still need to penetrate. You could drop below Ritual range by using an intangible tunnel. You could also avoid the Ritual with a device since it's just a Ritual due to high level, and then you could load on penetration if you work together in the lab.
one point is this- in most tribunals it is not considered to be scrying to check to see if something has magic resistance- especially someone who is known to be magical. if you check this at mid-day you can determine with a high degree of likelihood if the person is a hermetic magus or not, and how to proceed. I would think an InCo to check identity with zero penetration should suffice as well as give you the identify if they have no parma magica.
Also depending if in your saga someone can detect when their parma 'pings', as long as you do so with 0 penetration deliberately you can really annoy them...
Set up the blood in a target in the main hall of your covenant with a "If annoyed, throw 0 penetration non-damaging spells at this and know you are annoying one of our enemies..."
If he's a hermetic magus, being awoken at all hours or interrupted during lab time with "ping, ping, ping" essentially will tick him off, and angry people make mistakes...