1. When did this consonant shift happen in English? Etymonline mentions 12c.
Everybody has their own set of pronunciation habits. /ð/ can be pronounced in various ways and still be distinguishable because its existence is easily predicted/"auto-interpreted" by the brain. /d/ or /dð/ are common variants. It would be very hard to say exactly when such a shift occurred in the English language given that these transitions happen over time, unless an obvious answer exists like "The High Shift", but that's pretty unique, that's why it has its own name :P
2. What are some more examples of the /ð/ → /d/ in English words?
I'm not sure what "in English words" specifies in your question. I assume you're asking for words where the English /d/ takes place of the German /th/, i.e., words that are the 'opposite way', since words that illustrate the 'High Shift' are easy to think of/find.
Assuming you were asking for examples of /ð/ → /d/ -- so words where the English /d/ replaces the German /th/:
dollar / thaler (see note below)
For the rest of the list that follows, ignore the spelling of the German words, think
of the pronunciation instead. Of course this will vary from person to person, but
these are arguably at least semi-aspirated /th/ sounds as opposed to just a 't' sound.
Invariably though, this all depends on how hard you
enunciate the latter half of the sound. I'm not going to pedantically look up the
pronunciation guides in a dictionary, I have several relatives who speak German. Some
aspirate the /h/ part of /th/ quite heavily, and others barely at all. You decide
whether they count or not.
- devil / teuful
- day / tag
- door / tür
- deaf / taub
The Dollar / thaler pair is a good specific example of cognates with this particular shift. A Thaler is an old unit of currency from which "dollar" also gets its name. I mention this specifically in response to your question:
This means a shift from /ð/ → /d/ also happened, doesn't it?
I can't cite a particular 'shift' occurring in that direction, but the shift is definitely not one-way, perhaps they are simply commutatively understood and thus interchangable? If one were looking for evidence or trying to convince his peers of a 'reverse shift' having occurred, this is a pretty good lead-in that supports that theory.
You obviously know about the opposite way (high Germanic shift, English /th/ replaces the German /d/) but since I wasn't positive which shift you were asking for more examples of, I'll include some of this kind too:
- thank / danken
- leather / leder
- three / drei
- earth / erde
- bath / bad
- forward / vorwärts
- word / wort
[Again, for some of these...it's pronunciation, not spelling of course :)]
3. How is the shift mirrored in other Germanic languages?
Firstly the easiest thing to explain: Olde English ignored the shift completely. No effect there.
The fourth phase of the High German Shift, þ/ð→d occurred around the 9th/10th century, based on historical Old High German-Language books containing the old/unshifted version from around that time period. There was a lot more to the High German Shift than this particular mutation...although it was the most impactful & important. The first three phases of the High German Shift affected voiceless plosives, leading therefore to mostly isolated changes (High German only. Lower German was mostly unaffected.)
The fourth phase shifted the dental fricatives to /d/. This is distinctive in that it also affects Low German and Dutch. In early Old High German, as in Old Dutch and Old Saxon, the voiceless and voiced dental fricatives þ and ð stood in allophonic relationship (as did f/v and s/z), with þ in final position and ð used initially and medially. The sound ð then became /d/, while þ became /t/. This shift occurred late enough that unshifted forms are to be found in the earliest Old High German texts, and thus it can be dated to the 9th or 10th century. It took several centuries to spread north, appearing in Dutch only during the 12th century, and in Frisian not for another century or two after that.
(Can't post more than 2 links with a low post count apparently. So you'll have to 'manually fill in' what's obviously missing)
(Thanks Ruakh for fixing my links <3.)
Things to read:
Hope that helps.