Finnlines has instructions for foot passengers on its website:
It contains the following mention:
Please be aware that there is no walkway from the Lübeck-Travemünde
Skandinavienkai train station to the Skandinavienkai Terminal.
As a foot passenger you will be taken ...
According to https://www.iatatravelcentre.com/world.php, you can enter Germany because of an exception applying to "passengers with a long term visa issued by an EEA Member State or Switzerland."
While airlines will check IATA (Timatic, more specifically), it's prudent to check the official source which concurs. https://www.bmi.bund.de/SharedDocs/...
Is it possible to do something like that?
Yes, e.g. https://www.hertz.com/:
Is there anything I should be aware of?
The drop fee. https://www.enterprise.com/en/help/faqs/car-rental-drop-charge.html:
A drop charge is a fee for picking up a rental car at one location and returning it to a different location. The rental car drop fee varies depending on ...
On the page you are linking to, you can find in the section 'Fragen und Antworten ...' (questions and answers) a more detailed description about what they mean with 'mostly'.
International tickets can be used flexibly until 2020-10-31 with the following restrictions:
For all trains with compulsory seat reservations, you need a new seat reservation for the ...
Under current exceptional circumstances, you need to check relevant COVID regulations as well.
You will be required to undergo COVID testing in India or France. A mandatory quarantine may be ordered if the result is positive or you present symptoms.
Additionally, you will need to sign a certificate that you are symptom-free and give reason for your travel to ...
Short Answer: you can transfer airside at Frankfurt without a transit visa.
Longer Answer: Airlines check passenger documents carefully before boarding. The IATA database Timatic collects and presents this information to the airline, which is reviewed before a boarding pass is issued.
The German Federal Foreign Office presents different eligibility ...
Just from looking at a map of the area, my instinct is that this would be very tight. Probably doable with a taxi but not likely on foot is my rough assessment of the matter.
However, a more important question is, does it matter? What sort of train journey are you making? If it is a relatively local one (within Schleswig-Holstein, or to Hamburg) it is quite ...
The 'German' rules are, in reality, the rules of the individual Federal states (Länder).
As your first source also states:
Regulation of the quarantine obligation falls under the jurisdiction of the Federal Laender.
Both your second source (Nordrhein-Westfalen) and the corresponding Berlin law use the word aufgehalten, which in the English version of the ...