I am so sorry to read about your MIL and FIL. I do hope that they are okay. Do they have someone that comes in weekly to help them out? This may be something to discuss w/your h since he's out of state so much.
Your IC is correct about the "inability to make a decision" as symptom of depression. That's one of the reasons they put things off because they can't put one foot in front of the other when it comes to making decisions. Your h is in a lot of pain and it's going to take some time for him to recover from this. It's all about him and finding ways to cope and relieve the pain right now.
I have read about triangulation and it's definitely a tactic and one that people aren't even aware of when it happens to them. Yes, it's an ego trip for your BIL when he pits the two of you against each other. It makes him look like the bigger "man" in the situation. Now you have a good idea of what you are dealing w/and can handle the situation w/the BIL. Stop and think about it...he took great pleasure in telling you about the desk and what was in it. He was hoping for a heated phone conversation about the "treasure". He didn't get it.
During the crisis, personality disorders tend to "flare up", i.e., such as narcissistic tendencies. Think about it, a two year old wants what he wants when he wants it and if he doesn't get it, on the floor he goes, stomping his feet and holding his breath. It's being selfish during MLC and hopefully those tendencies will settle down once the crisis is over and done with. The same thing happens w/passive-aggressive behavior in MLC...it comes out to play even more so.
Spend as much time as you can w/your in-laws this weekend. They need you now more than ever. The garden will be there waiting for you when you return home.
Keeping you and your family in my thoughts and prayers.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.