How can I (US Citizen) get into Germany mid-June 2020?
Unlikely according to
Deutsche Welle (mirror):
Entry from non-bordering countries will be banned until at least June 15. This may change for some countries, particularly those within Europe's 26-member Schengen zone of visa-free travel.
Russia and the United States will be unlikely to see any change in their situation. Authorities singled them out as being particularly dangerous.
Also FYI IATA points to the Bundespolizei's website (mirror 1, mirror 2), which gives some pretty detailed information on the current covid-19 restrictions for people traveling to Germany.
As a side note, regarding your sentences:
The UK government is about to enact a 14-day isolation for inbound travelers. This must mean they are allowing inbound travelers.
Yes but not necessarily all inbound travelers. For example, Thailand has a mandatory 14-day quarantine but currently (2020-06-08) only allows 500 inbound travelers per day (mirror). But yes for the case of the UK, it is open to all inbound travelers according to IATA.
Update (2020-06-23): E.U. May Bar American Travelers as It Reopens Borders, Citing Failures on Virus