I'm open to all ideas, and yours isn't necessarily wrong - but I don't think
a case can be made for it in the way that you do. You have already had some
replies about vocabulary, and you say that you are primarily basing your
ideas on grammar. But even here, the case is very shaky. I don't think the
grammatical elements you point to actually exist.
You mention four aspects of grammar:
>Germanic languages' much greater tendency to agglutination,
>Germanic's better preservation of IE ablaut,
> adjectives come before nouns,
>leveling of past tenses relative to other IE languages.
None of these are secure; the first two are chimaerae.
Agglutination. Not present in Germanic, unless you mean compounding, in
which case you need to see what happens in Greek and especially Sanskrit,
which can make compounds that make your hair curl (though admittedly, the
extreme examples are in the later language)
Better preservation of IE ablaut. This is best preserved in Greek and
Sanskrit, though traceable in other languages as well. Germanic does not
preserve it better than any other branch, due to analogical levelling and
new formations in adjectives and nouns. Ablaut patterns in nouns are best
seen in Greek or Sanskrit, though also traceable in Baltic (especially
Lithuanian) and Slavic accentuation.
Adjectives come before nouns. This is also the usual pattern in Greek
("the big house" has the same meaning as in English, while "the house big"
means "the house is big"). Latin shows more flexibility, but adj before
noun is still the commonest word order (check Gildersleeve & Lodge section
676). The position of the noun seems to have been free in PIE.
Greenberg has claimed as one of his language universals that the position of
the adjective is related to that of the object pronoun (obj pronoun before
verb + adj after noun, and vice versa; compare French and English). I have
grave doubts about this, but it is possible that patterns of word order are
related to each other in some way. At any rate, you cannot claim that
Germanic is the only IE language to have this pattern.
Levelling of past tenses. It is still unclear whether Hittite and Germanic
"levelled" pre-existing patterns of past tenses, or whether Greek and
Sanskrit developed their diverse patterns without the other IE languages.
There is a growing consensus that Hittite does preserve an older system.
Germanic on the other hand shows precisely the same inherited elements of
verb formation as the other IE languages (except for the notable absence
of -s- forms). Latin and Germanic have both reshaped their verbal systems.
So I don't think you can claim anything unique for Germanic here.
Peter