Regarding question 2:
Generally speaking, a country like the United States can regulate how their own citizens, residents, and companies trade with other countries.
- They might ban their citizens from the import of specific items, e.g. Cuban cigars.
- They might ban the heir citizens from trade with specific people or companies, e.g. certain Russian businessmen.
- They might simply ban heir citizens from all trade with a country.
If a ban on trading with a company or country applied only to direct trade, it would be worthless. So while Cuban cigars were barred, it did not matter if they were coming directly from Cuba or if they got re-packaged elsewhere.
When the US get really serious about sanctions, as it was regarding Iran and North Korea, they would go even further. They would ban the trade with a country, and the trade with any company which trades with that country, and the trade with any company which trades with a company which trades with that country, and so on.
Doing that is a pretty tedious effort, but it forces everybody to take sides -- trade with (or in) the US, or trade with their enemies. That would leave German companies with the free choice of not trading with Russia or not trading with the US. Most would opt not to trade with Russia in this case.
The German companies might also complain to their government, which may or may not take the issue to the American government. While the US is generally free to regulate their international trade, this may be construed as a violation of other trade agreements which the US has signed previously, and it might be seen as an unfriendly act by an ally.
Getting Europe to go along with sanctions on North Korea is relatively easy. Doing the same to Iran was harder. Getting a consensus on Russia sanctions may be harder still.
It is my estimate the Russia and Western Europe have achieved a kind of MAD regarding the energy issue -- Russia really needs European money, Europe really needs Russian gas, and I doubt anybody knows who would collapse faster if the energy trade gets seriously disrupted.
The number and location of gas pipelines really matters. For instance, as long as the Soyuz pipeline through Ukraine is used for shipments to Western Europe, it is difficult to cut off shipments to the Ukraine -- if their citizens are freezing, they can take gas now and worry about payments later. New pipeline construction can shift the power balance.