I must agree with Lexie here... this was an act of violence against you... and to verbalize that he thinks you drove him to it solidifies that. You did do all the right things.
You are probably in shock. When I've been through things like this, my first coping mechanism is to throw myself into the "normal" things like chores... its how I try to make things seem to myself as if the trauma didn't happen. As sick as that may be, it works for me at first, to get me through the initial shock. Then, like you are going through now, when things are more manageable (ie... I have all the facts and some sort of temporary solution is achieved) I begin the "falling apart" stage. I cry. I used to try to suck it up, but crying has become like a natural tranquilizer for me... if I see it through and don't try to suffocate those feelings my eyes are trying to expel.
I've been through similar experiences, the last of which was my ex-hole putting my 9mm to his head, the trigger of which he never pulled. He is, and was, crazy enough to do it. It threw me into Post Traumatic Stress Disorder but good. Of course, he told me it was my fault too. I'm no longer a recipient of his "blame game", he'll have to try that on his cellmate or something now.... he went to jail for beating up the mother of his kids when I had to evict him from my life... and he'll be gone for a few years now. Not that I care anymore.
If I was you (and I know I'm not), I would take this "moment of silence" to confirm the boundaries I already set with myself and him. I would continue with the plan of action I already had. I would thank my higher power for a break in the daily incidents of having to deal with him, and I would find a way "not to care" about what his brother was talking about.
Believe me, this will not be easy. Its nearly impossible not to care at all for someone you once loved so much. But it will get better if you don't allow his attempt at pulling your heart strings (yes, some people think "if I was dying she/he would come back to me") to work.
In my experience, people who want to kill themselves, don't talk about it before they do it. Those that are trying to obtain something other than death (and lots of time its help, but not always - sometimes its for effect, and if someone wants to get mad at me for saying that~fine, but I been on both sides of this) will somehow let someone know what they're feeling before making even an attempt at harming themselves, or very soon after they realize that their actions in the state of mind they were in when they took steps toward suicide could actually take their life.
Your husband could be any of these, he did make a cut before you got there.... on the other hand, he did do it in the shed with someone doing yardwork, and you just having gone to call the cab, and he waited for you to show up in the shed to cut deep (effect?)
He's going to be evaluated now, you don't have to try to figure this out, you can't live in his mind... and you sure don't want to let him live in yours. You sound like a very strong and intelligent person. Time with your therapist will help get you through this, you are already taking care of yourself, keep it up. You'll get through this.
Hugs,
Lori