既然提到從字典的觀點來看,而且又提到牛津字典(http://www.oup.com/oald-bin/web_getald7index1a.pl),我就補牛津字典查詢in和here的東西來瞧瞧:
in:
很抱歉,裡面完全沒有in here/there/where的範例與用法.

here:
裡面介紹了here的各種用法,其中唯一有in here的,卻是一句被列為"slang"的粗話.
get your ass over / in here, etc. (slang, especially NAmE) a rude way of telling sb to come here, etc.—more at cover v., kick v., kiss v., pain n
所以參考牛津字典之後,如果要拿這種東西當成"in here"可到處使用的圭臬,我也無話可說了.

至於把here當成名詞的部分,牛津線上字典沒有,但我剛好以前有買東華出版社出的英英/英漢雙解牛津字典,裡面對於把here當成名詞(n.)的用法,就是here擺在句首(抄錄原文如下):
(with front position, and inversion of the subject and finite v if the subject is a n, but not if the subject is a pers pron).
而其在東華這本牛津字典英英/英漢雙解所列舉的用法,也完全沒有"in here"這種東西,就更別說『在"in here"中,in是介係詞,here是名詞』的這種說法了.

另外有提到對岸網友把範例句子裡的in here,解釋成是同位語;但是同樣在牛津字典裡,同位語是用在名詞或名詞片語上,所以如果要說這也是同位語,也許態度上該有所保留.原文語連結如下:
apposition
(grammar) the use of a noun phrase immediately after another noun phrase which refers to the same person or thing.