No, you are almost certainly mistaken about any overt borrowing from German Obstgarten to produce English orchard.
You are struggling too hard to look for an added -r-, when in fact, it was already present in Latin (h)ort-(us) right from the get-go. There were also cognate Gothic words starting with aurt-.
However, there is nothing resembling the German word Obstgarten.
Regarding orchard’s etymology, the OED states the following:
orig. OE. ort-ʒeard, parallel to Gothic aurti-gards garden, the first element of which is considered to be Latin hortus (in late and medieval Latin ortus, Italian orto) garden. Cf. Gothic aurtja gardener, and OHG. orzôn ( :– *ortôjan) to cultivate.
Already in 9th c., OE. ortʒeard passed into orcʒeard, orceard, whence ME. orchard; also, with recognition of the second element orch-yard, ort-yard, or, with later conformation to Latin hortus, hort-yard.
The word has had many, many, many, many, many — did I mention many? — spellings in its history since King Ælfred the Great first wrote about it in 897 ᴀᴅ.
The OED lists 44 (yes: forty-four!) different historical spellings of the word, divvying these many forms up into four main sets, denoting each set with a Greek letter and with numbers indicating the nth century (and “1” means from before 1000):
1 ortʒeard, ordceard,
6 ortyerde, ortȝard, ortiard, (ortesyerde),
1 orcʒeard, -ʒyrd,
(1 orcird, ‑yrd, ‑erd),
3 orchærd, (horechard),
5 Sc. orchart,
6 ortchard, north. orchert, ‑erit).
4 orchiard, orcheȝerde, ‑ȝarde, ‑yerde,
5 orche-ȝerd, orcheyerd,
6 orchiarde, orchyarde, orcheyard,
6 horteyarde, hortyeard,
7 hort-yard: see hortyard.
Not a single one of those 44 historical forms bears any real resemblance to German Obstgarten, except insofar as the second element shares a common ancestor in Primitive Germanic with the second element of Modern English orchard. That second element was gart in OHG and geard in Old English, a word that also gave rise to yard in English, and which is closely related to the current Northern English word garth from Old Norse garðr.
Given this evidence, I believe your notion that Obst- contributed to orchard is wholly unfounded. All historical records stand against that idea.