YAC: a couple of questions

Hi, guys!

Merry Christmas to all of you. In these days of vacation I'm wondering about
a new conlang. I haven't chosen the name yet, but I have a bunch of ideas
I'd like to experiment. Thus, I have a couple of questions to submit to your
scrutiny.

1) Since I don't like things as Latin 'consulibus illustribus', where the
desinence -ibus is redondant and baroque, I'd like to avoid these
allitterative processes which can be engendered by case endings. For Vaiysi
I've worked out a cool and rather elegant way to do it: in a noun phrase in
the locative or in the allative case only the last membre of the phrase
inflects, this because these cases come from pospositional construction
taking the absolutive case:

lyaskam samam
good.DAT man.DAT
to the good man

miylinini rinani
little.GEN town.GEN
of the little town

but:

hyene talou
beautiful house.LOC
In the beautiful house

which is from Suiméni (Vaiysi ancestral lang):

séne talu ó
beautiful.abs hous.abs at

or:

sile burmouved
sky cloudy.ALL
up to the cloudy sky

which is from Suiméni:

sile burmówe it
sky.abs cloudy.abs to

Now I'd like to have a system looking and working more like in German, where
adjectival and nominal inflections take different endings: das schöne Haus,
des schönen Hauses, ein schönes Haus, schöner Häuser, schönstem Hause etc.
Where are these endings from? How did they generate?

2) If I want adjectives to work as verbs (as in Japanese or Arabic, i.e.), I
will translate 'the red car' as 'the redding car'. But, since my lang will
be inflective, shouldn't the particle take the endings of nominal
declension? I mean: let's assume 'yum' means 'car', 'sieag-' 'to be red',
'-ul' is the particle's ending and '-im' stays for the genitive case The
phrase 'of the red car' will be:

sieagul yumim
red.part. car.gen

or:

sieagulim yumim
red.part.gen car.gen

What if the particle is not properly a particle but a contract relative
clause (thus 'sieagul' doesn't properly mean 'redding', but 'which is red'),
as Chinese -de adjectives (mang de ren = busy people)?

Then I should translate the car's exemple as:

yunim sieagul
car.gen red.rel

Are there languages which do this?

Thanks in advance.

Luca