Hi ! Meet again here, let me contribute a bit here
0) Of course there's an impact, a more cleaner div will get the overall filesize smaller and it make the webfaster, this also applies on the render engine. BUT.. you're we're talking about less than a second's impact here
1) No, Not directly, eventhough seblod has capabilites to use all joomla table, but i tried to completely using external table and it failed to work (especially with list)
2) Yes, with seblod you can create a web without even have to code. BUT you still require PHP, HTML, CSS knowledge for your advantage coz not all the feature is 'out of the box'. Also with seblod you can use conventional PHP code without joomla MVC
3)In My View, Upgrade from JSEBLOD to SEBLOD is bit a mess here.. the better solution is to hire an assistant , or ask your staff to copy and paste manually.
4) There's Export Add-on in the market, have you tried it ?
5) Tons of discussion, i think the core dev would leave the CCK aspect to 3rd party, so they can focus on core module and make joomla generic for multiple purpose