I'm not sure the perceived or actual "decadence" of Austria-Hungary played much of a role. More likely it seems that massive divisions between the Austrian and Magyar aristocracy, and the general weakness of the state and its institutions in the face of these divisions were the major problem. Although I do admit that this area is not my area of expertise by any means.
1. Seems generally correct.
2./3. Could Germany provide for Russia the financial support that France did OTL?
4. One issue before we get to war is that France will obviously do more to prepare in the face of a world where it is facing both Germany and Russia. I can see a lot of France financing to both Austria and the Ottomans, and maybe Italy and others as well - France effectively building a larger alliance, with British help, to contain German, Russian, and Serbian ambitions on the continent.
5. Probably, but the UK could probably wreak havoc on Russia in the East once again, especially with the help of Japan, no?
6. An alt-Crimean war, but on a grander scale, I suppose...
7. I see Italy preferring to remain neutral in this case - their joining the Central Powers would presuppose all the successes that you assume they will have and I'm not sure those are certain by any means.
8. If the Central Powers win a quick war then no, the US wouldn't get involved.
It's not mine either. However, I meant decadence in a sense that AH was no longer a top power. I mean, Austria was a great power, but it was well behind Russia, France, Germany, the UK and the US. My sources (mainly books and the youtube channel "The Great War") make me think that Austria-Hungary's main problem was the lack of modernization, in both tactics and equipment, and, as you have already said, the division and weakness of the state.
2./3. Germany CAN provide the finalcial support, thanks to their powerful economy. The question is, would they?
4. I didn't realize that in such a situation France would try to prepare itself better. Who knows? Perhaps such a France would be able to even win against Germany... but then what? Should France exhaust itself with Germany, defeating Russia would be impossible. As for support to allies... I don't really think that France can solve the core problems of neither AH nor the Ottoman Empire, but their support would be very valuable. A well equiped, trainned and led Ottoman Army may get the upper hand in the Caucassus, but I still think that AH would collapse, and once the full might of Russia is in the Caucassus, the Ottomans can't last that long. As for Italy... aside from some German colonies, I don't see what else they could win in a war against both Germany and Russia, so I think they would either join Germany or remain neutral.
5. Yes, but Japan can't do much on its own, except destroying the Russian Pacific Navy (the Baltic fleet would have no chance of going there thanks to the RN). As far as I know, Japan was in the border of a socio-economic collapse during the Russo-Japanese War. Thanks to the trans-siberian railway Russia doesn't need a navy to supply its forces, and I'm sure they have the manpower to spare. The IJN might have been great, but their army was not that good actually. Also, though loosing Vladistock would be bad, the rest of Siberia and the Russian Far East has little strategical value and marching from there to Russian Europe isn't something practical (you can oonly do that if you're the mongols). I'm sure that the RN's help would make landings and capture of ports easier, but the main core of Russian Population and industry would still very far in the west.
6. With Austria to take care of this time.
7. Of course such a grade of success isn't certain, but I was working under the assumption that the threat's author wanted them to join as well. Remaining neutral would be the wisest choose, but should Austria collapse and with the entire French Army north, Italy would have a chance to win almost all the lands they desire in a war that would seem to be almost over.
8. That's for certain. By the way, I don't think that a German-Russo Alliance would be called "The Central Powers". Maybe something like "the two emperors' league".