I have just gotten every multiplayer achievement and campaign achievement in general for this game. You are still able to log into live and even though you cant find any multiplayer games you can have a custom game mode with up to 4 players which is all that is required for every achievement.
Please check my gamertag and date I earned the achievements for proof. If you would like to know how I accomplished this just send me a message over xbox live and I will help walk you through it as I am not computer savvy at all and was able to work around a custom game mode to get the multiplayer specific achievements with the help of some people myself.
1. Since I have played halo 2 soooo many times through all the other versions I did not bother beating the game on any other difficulty other than legendary as this will unlock the other difficulties with it as per normal halo games.
2. I used the keyboard for everything since I dont know how to hook up a controller to my PC and everything went fine although with the keyboard this was the second hardest halo 2 legendary run ever for me, especially since my computer is old and pretty laggy. Only time Halo 2 was harder was when I did the Laso run in MCC
3. You do not need to do the campaign on legendary twice to get the Legend achievement. Once you beat the game once on legendary just jump in the red beam on the custom map (forget the name) and it will unlock for you. Any other suicide does not.
4. I bought a brand new copy of the game off on Amazon on April 23 2017 and it took about 5 days to get to my house. All you have to do is install the old windows 7 version for Games for Windows live (I was able to use the actual link from the official Microsoft website) and then you will need to update it about 5 times. I was worried at first it wouldn't work because I kept having to log in and a few times it wouldn't log me in until I tried again and then an update would pop. Once all the updates are in you will be logged into live and can see that your achievements are tracked on xbox 1 and so forth.
5. Be sure to watch some youtube videos on easy ways to beat the Heretic and Regret boss. This would have been a nightmare without the help of those easy ways to win the fights.
6. You will not find any outside players to get the multiplayer achievements since the main server is down but you can click "Live" from the main menu and have a custom game with up to 4 people. This will be a bit tricky since you are playing against yourself but most are easy to get, overkill will be the hardest since you have to kill your three guests and then find the fourth as he re spawns in time to get the overkill.