How can I (US Citizen) get into Germany mid-June 2020?

The current restrictions (in place until 15 June 2020) do not allow you to enter.

If you are not a citizen of an EU member site (including United Kingdom, Iceland, Liechtenstein, Norway and Switzerland) and not a?country national with a long?term right of residence in an EU Member State and/or the countries mentioned above you have to be travelling for one of the following reasons to enter Germany:

• healthcare workers, health researchers and care professionals;
• cross?border workers, transport personnel employed in the movement of goods and other essential sectors
• diplomats, staff of international organisations, military personnel and humanitarian aid workers insofar as they are performing official duties
• passengers in transit
• passengers travelling for imperative family reasons persons in need of international protection or seeking entry for other humanitarian reasons

As you don't meet any of these reasons, you can't enter Germany. After the 15th the rules may change, this is unknown yet.

The European Commission has just (2020-06-11) announced suggestions for the possible lifting of the temporary restriction on non-essential travel to the EU after the 30th of June 2020.

Given that the health situation in certain third countries remains critical, the Commission does not propose a general lifting of the travel restriction at this stage. The restriction should be lifted for countries selected together by Member States, based on a set of principles and objective criteria including the health situation, the ability to apply containment measures during travel, and reciprocity considerations, taking into account data from relevant sources such as ECDC and WHO.

For countries towards which the restriction remains in place, the Commission proposes to enlarge the categories of permitted travellers to include, for instance, international students.

Details are given in the checklist, which can be downloaded as PDF.

Suggestions are also being made for the Resuming [of] visa operations.

This information should be useful as an overview in which way things are developing and a possible timeframe (nothing more).

Sources:

• 2020-06-11: Commission recommends gradual lifting of travel restrictions
  • Suggested Checklist to be used for the possible lifting of the temporary restriction on non-essential travel to the EU (PDF)

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