Answer It depends what on your making. For example
You can use the wavy layer in between as sheet metal (Great for makeshift barricades),
You can cut out a few layers and lay em on top of each other, then use greenstuff to cover up the gaps where you cut it to create a hill.
Look up "Warhammer 40k Scenery" on YouTube and check out what they've done.
No, in Warhammer 40k, players are generally not allowed to mix factions when creating their army. Each faction has its own unique units and abilities that are meant to be used together for balance and thematic consistency.
No, the Dark Angels are a chapter on their own, and the are regular Space Marines, servants of the Emperor who haven't fallen to Chaos. Their name is misleading however.
There are no rules surround it, or stopping your from doing it. In tournaments, it wouldn't be legal, but so long as you give your race a decent background and agree on rules to use with an opponent it should be fine.
Making your own chapter is quite easy. However, making one from the pre-heresy era is not recommended as all the chapters then were known and numbered. Don't use the weapons from other races in your chapter, but you can make some good models from parts or equipment from them, just stay away from their weapons. Don't make the weapons to are different to traditional Marine weapons.
Does your computer meet Minimum Requirements? Is the game patched to its newest version? Did the game install correctly, if unsure run a defrag, uninstall and reinstall or verify integrity of game cache if you own the game through Steam.
Dungeons and Dragons miniatures are just like the roleplaying game, except they give you a visual reference to what you're fighting/playing as. The miniatures, although really cool, are pre-made unlike the roleplaying game where you make up your own character(s).
Your own choice.
Rather a lot. The best way to find it is to buy it and do a line by line comparison with the old one. However, if you don't feel up to that kind of effort, you could browse 40k forums, quite a few users posted their own summaries of the most important rule changes.
35 levels or days
most hobby and comic shops carry them unless they are owned by a company who makes their own products. however wizards no longer makes the star wars miniatures and george lucas has (to my knowledge) not yet made any contracts with any manufacturer to make new ones.
Not enough... Average about 30-40k / year unless you have your own company.
However you want, look at pictures of them for some ideas. Go to lexicanum for some ideas on which Craftworld coloration you'd prefer, if none meet your fancy, make your own!