lemme help ya buddy cause i play this game for 2 days hahaha.
in order to get Walter, you need to post a rose on your social media so mark will be crazy jealous, and make sure adam gets beaten up by mark. (and ignore mark's text, don't block him! even when the stalker calls/text, ignore them too.) and then adam will transfer to another job branch, which means you will work with walter. then after a while, you will get a scene of walter asking you out for a drink (i hope i am right about this cause i play so many times and i might get them mixed up :")
in order for adam to call you during mark's route, you need to have allie's death, adam will come to your house to check on you, make sure be honest with him about the text and admit you're not okay, and during mark's route, adam will def call you.
p.s if you want adam to come over, make sure to select the "i stayed away from him" and it will trigger the scene (in case you have trouble cause i know i did lmao)
also, this was my experience when i played the game, so idk if you can get the same scene with different choice.
let me know if you don't understand or is stuck cause tbh my brain is mashed atm HAHAH