You need to download a recent development build (trunk). You can get almost daily builds for each platform at http://builder.blender.org Glad you like it!
And you are right, my bad ^^ Don't know if you use my setup to edit or if you use the properties, but a popup with keymaps could be nice to edit on the fly.
You mark the seam edges and lscm unrwap takes care of things :).You don't need to add a modifier you can mark the edges in 3d view when you want.Max 2012 has the same exact unwrap type added (lscm).
Yeah, it depends on what mode you are in. In global mode, pressing two times puts you in local mode. In local and any other mode pressing two times puts you in global mode. I don't think you can remap this.
You can't paint vertex alpha but you can paint a vertex group and use that as an alpha. It's a known limitation and I hope they get rid of it but it shouldn't keep you from doing anything especially if you're using any render system that makes use of nodes where you can get any vertex group weight you want.
for target-weld : under the W menu you have merge selected. this gives you an ugly pop up menu each time which asks you how you want to merge but you can bin a specific funtion to any hotkey i´ve bound merge -> last to active to Q and weld at center to shift-Q