To be or not to be in the Dark Brotherhood and a Guild Master

First of all I'm trying to avoid repeating this question: https://gaming.stackexchange.com/questions/40039/what-should-i-consider-when-deciding-to-join-or-destroy-the-dark-brotherhood

I was reading http://elderscrolls.wikia.com and I have a few doubts about the history line. I read that you can kill Astrid, but then you can join the Dark Brotherhood anyway, if you do not choose to start another history. Is that correct?

My idea is to try to complete this questline but then kill 'em all. So I think I'll kill Astrid first.

My questions are:

1. If I kill Astrid can I really join the dark brotherhood and then once I've completed that questline destroy 'em all?
2. If not, is there a way to do that?
3. If I only choose to destroy them, can I get the Shadowmere? How?
4. Is there a consequence for being a member of the dark brotherhood with any NPC?
5. In relation to these, I'm married with Mjoll the Lioness. If I become a Guild Master will I have problems?

I'd be grateful for any comments or answers. Thanks and kind regards.

Please before you downvote this question, comment or edit why you think this question should be changed.

You can only choose one of two things:

1. Kill Astrid and destroy the Brotherhood (and not obtain shadowmere)
2. Join the Brotherhood

Also, there are barely any consequences. No one will really have anything but maybe different speech options for intimidate.

Of course there always are console commands to fix your needs if you feel like cheating.