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A few quick answers: hopefully others will refine, elaborate or correct:

1. If you only intend to work in Germany, and assuming you have a university degree, a CELTA may not be absolutely necessary from the point of view of getting jobs, and compared with the expense of the course. But it's very worth getting if you want to teach in other countries in future. And a good CELTA course will actually teach you something useful.

2. The problem with online TEFL courses is: there are so many bad ones (where the certificate requires negligible acquisition of knowledge and skills) that employers may well assume that any online qualification is useless. So even if you find a good online course, it may not help you find jobs.

3. As well as the CELTA, consider the Trinity College London CertTESOL. Their site lists a 4-week full-time course in Moenchengladbach, which may be within your range. See http://www.trinitycollege.co.uk/course_details/viewcoursedetails.php?location=germany&course_id=1317 or the Eckersley School of English at http://www.tesol.de/.
From what I have heard, Trinity courses are just as good as CELTAs (from the point of view of how much you learn), and they should be as highly regarded, but they sometimes aren't.
NB I haven't done a Trinity course, and know nothing about the Eckersley school in Mönchengladbach.

... or indeed at International House, Cairo. Rather requires leaving the Köln/Bonn area, though.

I don't know the London Teacher Training College, but from their website it's clear that they teach and test their own certificate. It may well be that their teaching is good and their testing is rigorous, but from the point of view of investing in a certificate that will always be internationally recognised, you're only really safe with the Cambridge CELTA (http://www.cambridgeesol.org/exams/teaching-awards/celta.html) and to a slightly lesser extent the Trinity equivalent.

Good luck with your search. I hope somebody else will offer a useful suggestion.