I don't think we need to worry about the one 1655 citation as a noun at all. I wouldn't even list a noun sense as obsolete; it would only be confusing, unless someone can find an example of it in something that's still read widely, like Shakespeare.
It's important not to confuse that sense, which is a countable noun that could be called 'an intricate' (today: 'an intricacy'), with the nominalized adjective. All of Ecgberht's examples, without exception, are adjectives, as wupper already pointed out:
>>Im Prinzip kann (fast) jedes Adjektiv so substantiviert werden: the beautiful, the mysterious, the hungry usw.
So they have nothing to do with the OED noun entry, and there's no need to list any of them as separate dictionary entries.
If you did, as wupper said, it would be 'das Komplexe.' And you would need to list all the other choices too: das Knifflige, das Komplizierte, das Schwierige, das Umständliche, das Verschlungene, das Verwickelte, das Verworrene, das Verzwickte, das Aufwendige ...
No other dictionary I've ever heard of does that, for reasons that are surely obvious.