I'd say that your German colleagues are mishearing the English pronunciations.
The German letter ü makes the sound [y], which does not occur in English.
The words loose, poodle, food, and most other words with oo have the vowel [u], which is usually spelled u or uh in German. Historically this is a long /o/ sound that was written with "oo", the pronunciation of which has shifted to [u] as a result of the Great Vowel Shift.
Some words with oo have instead the vowel [ʊ]: good, hood, book. There is no rule that predicts which words have this pronunciation, so you have to memorize it. The [ʊ] sound occurs in German as an allophone of /u/ in closed syllables. The vowel [ʊ] is shorter, more lax, and slightly centralized relative to [u]. This sound also tends to come from an older long /o/, though the reasons for this split are complicated and obscure.
A very small number of words with oo are pronounced with an [o] vowel: door, floor. These words always end in r, because the final r colors the preceding vowel. This is the same sound that is spelled o or oh in German.
No, there cannot be. Phonemic /e/ at the end of a word in English can only occur as a phonetic falling diphthong [ej], as in say or they. That’s why those have a ‹y› in our spelling today, and why sleigh has an ‹i› in it.
And unstressed /ɛ/ will soon enough go the way of all things, despite what bokeh enthusiasts would have you believe. (Same with the meh-sayers.) Because English phonotactics forbid an open /ɛ/ at the end of the word, those will therefore soon enough become either a phonemic schwa /ə/ — or else become a close vowel like /e/ or /i/ phonemically and so one with the characteristic falling phonetic diphthong [ej] or [ij] required by English phonotactics.
I therefore little doubt that the word currently spelled bokeh will end up /ˈbokə/ just like the boca heard in the city of Boca Raton, Florida, to rhyme with mocha.
The other two possibilities are for bokeh to wind up rhyming either with hokey or else with hockey. This would be like how Spanish chile which ends with /e/ becomes in English chili which ends in /i/.
Only if the second syllable became stressed could bokeh become /boˈke/ or more likely /bəˈke/, which is the sort of thing you get when in English you pronounce the Spanish word olé under English phonotactic rules.
Whatever happens to words like bokeh as they assimilate to English, they will need to be respelled to use a spelling similar to whatever words they end up rhyming with in order for them to be predictably pronounced by monoglot English readers.
Probably spelling what is now commonly rendered bokeh in English as boka would have been better from the get-go.
Best Answer
Yes. In so-called non-rhotic pronunciations of English (which includes what are perceived as 'standard' British pronunciations), a written 'r' does not actually represent an 'r' sound when it is syllable-final.
On the other hand, the case of "iron" is simply a rare exception.