Glad the videos have been useful. As you learned in the videos and notes, you need to focus on Linus' behavior continously to make the most progress. In your question you refer to his behavior degrading when walking together. So, what you need to do is go back to basics when this occurs.
To do this, figure out what potential rewards he wants. This might include examples such as toys, treats, pets, verbal praise, a quick run, a game of tug, access to a location in the distance, or access to one of you. Use these rewards more frequently at the beginning when you are walking together and then as he shows more reliability use more verbal praise to motivate him to "keep going" before another reward occurs.
This is shown in the Intermediate Leash Walking Video. If he pulls, stop or go the other direction. You can wait until he stops pulling or gently guide him into place and then click and treat or say, "Good" if you are not using a treat and continue walking. As shown in the Basic Leash Walking Video with the toy exercise in the front yard, you can also wait until he looks at you as another way to get him to focus on you more instead of pulling.
One suggestion would be to have one of you stand 5 feet ahead of the other one that is walking Linus and practice a session with many repetitions where the trainer stops walking if Linus pulls and continues walking if he turns around or comes back to your side. You can wait him out or gently pull him back into place. As you do more repetitions, increase your expectations and get more of the behavior before he is allowed to say, "hello" to the other person. And . .. don't forget. Talk to him a lot when he is doing well. That can make a big difference. Let me know if you have more questions.