You have several facts incorrect, let me list some of them-
Austria married Marrie Therese to Napoleon because they did not want Napoleon to marry a Russian princess...telling.
The Hapsburg's were old past masters at the art of marriage-diplomacy to protect themselves temporarily; it was done as the HRE had been dismantled in the peace treaty by Napoleon and as a desperate attempt to keep the balance lands.
Emperor Franz later on gave a 'cold shoulder' to his daughter. Daughters and younger Sons did not matter, Empire alone was what mattered.
The Prussians weren’t much into this marriage-diplomacy.
Why Russia didn't agree for the marriage is because Russia wasn't desperate enough or rather Russia had never lost. Sure, they lost a few thousand, doesn't matter, the TSAR of All Russia was the Imperator and Autocrat of all Russia ordained by GOD, he wasn't going to let his sister marry an upstart Corsican Godless Corporal (as per the Tsar) who had gobbled the territory of France.
(As per Napoleon version) –
Why should I marry a defeated person’s daughter when I can marry my most powerful Adversary’s sister?
Oh (those old Kings think I am not a King), let me marry the longest ruling dynasty of Europe.
(You are looking at things from a 21st century prism, not from 18th and 19th century politics and ideology)
Austria, Prussia and Russia - rather than spending their energy sending troops against revolutionary France - spent the early 1790's splitting up Poland because they knew its value to their security and did not trust one another.
The 2 major partitions of Poland were done pre 1792, only the 3rd and residual partition was done once the war started.
The Prussians did send an army in 1792, it was stopped at the last moment on the French borders by Jean Moreau (the best French General of that time) & Kellerman (a German in French Service, these kind of things were very common pre 20th century, most countries in Europe used German mercenary soldiers and officers, Eg: the most Celebrated Military Commander of France before Napoleon was Moritz von Sachsen – Frenchified as Maurice de Saxe, 3 out of 4 major Russian commanders in the Napoleonic wars had names like – Wittgenstein, Benningson etc) and other Revolutionary Generals (Napoleon did not play a big part here) - see the 1st coalition for more details.
The Austrians and Russians together mobilised an army under Suvorov in the late 1790s (2nd Coalition), unfortunately the typical bickering that happened in an alliance took place and this whole expedition floundered. This is not just 18th century alliances, it had happened before and after, in the 1750s the Russians, Austrians and French failed to combine and defeat Frederick the great, in the First World War, the Triple Entente failed to defeat the CP (basically Germany + Allies), alliances are often tricky and each partner had their own priorities.
(BTW this is shown by the extra command points it costs to include an "allied" corps in your army unless Multinational Commanders are used- few and far in-between and again - This can be further re-enforced by reducing the % chance of MTSG between "allied" armies ).
If you are playing France will you invade Spain? Russia? Why would you invade Spain? An argument could be made for the enforcement of the Continental System vs. Britain but it is a poor one. In this case the ultimate question is: Should you be penalized VP for not invading Spain? I would really dislike that. Also, why not invade Russia up to Smolensk and the Baltic states and return the ancient boundaries of Poland? The Ukrainians and Balts flock to Poland. (Anyone ever see the footage of 1944-5 of civilians fleeing west towards the Nazis away from the Russians?) Should you be penalized VP for not invading Russia?
Now, this is what i call total misreading of the situation.
Spain was ruled by a Bourbon ever since Charles II von Hapsburg died, this meant France and Spain were traditional allies since early 1700, after the Battle of Trafalgar, the Spanish realised that the chances of winning a Naval War was more or less finished, this meant England will remain a powerful enemy, at this time England was ruled by the Young Mr. Pitt one of the best PMs ever and certainly the best Chancellor of the Exchequer, this meant England’s coffers were full and subsidies were ready to pour out to any current and future allies. Charles IV de Bourbon the ruler of Spain realised there was a better option to ally with England against France and this precipitated the war of France and Spain.
If you do not want War- win the Battle of Trafalgar!
Why was Spain Invaded? Napoleon was building an empire, it was built by taking lands from the Dutch, the Hapsburgs, various German states, Italian states, Spanish etc, if one part of the Empire rebels successfully, soon all parts of the Empire will rebel and a fresh coalition will be raised by the English, leading to destruction of the empire NAPPY built. There is no stop, there are only 2 alternatives on that treadmill-
1. Kill or destroy all enemies – Like Nappy did
2. Build a defensive so strong it is impregnable – Again he tried, not successful.
Secondly, stopping at Smolensk? Red Army? (Are you serious now?) - the Russians themselves did not like Communism or Stalin, moreover USSR was mostly ruled by a non-Russian; Ukrainians like Brezhnev, Georgians like Stalin, several Jews etc. Russian Empire was not Communist USSR (please do not try to impost 21st century thinking on this period, It was not AXIS OF EVIL).
Ukrainians and Balts liking Poland? Get real, they hated Poland- see the "times of troubles history of Tsardom of Russia post the death of Ivan Grozny (translated wrongly as Ivan the terrible) Why would they want to be in a Polish Schlacta rule? Also the Baltics was dominated from the 12th-13th century onwards by Baltic Germans (they were mostly LUTHERAN not CATHOLIC) who formed the socio-politico-cultural-military elite of that area and also of Russia from 1750-1917, they were the descendants of the Teutonic and Livonian order, do you really think they will like Polish Rule?
Ukraine was dominated by the Military colonies established by Catherine the Great, most towns and cities had retired military personnel and Cossacks, both of them absolutely hated Poles, Jews, Catholics etc. (the word POGROMS come from this era, does that ring a bell?)
Russians (including Ukrainians) followed RUSSIAN ORTHODOX CHURCH which considered CATHOLICS as type of HERETIC.
(Again i guess you are far too much in 2015 anti-Putin propaganda mode)
Why did European Monarchies declare War?
I guess you have never read 17th and 18th century History Books, if you did- recollect what the French did - - Overthrow of the legitimate regime, mass guillotine, regicide, destruction of the church etc. This totally horrified the European Monarchs, suddenly it seemed nothing was sacred, it was all nihilist and destructive. STATUS-QUO was the most important thing for a monarch, any change was to be stopped unless it was directed by the Monarch himself.
Many other things but i guess enough to send the message. I would suggest reading some Russian books.
Rascals, would you live forever? - Frederick the Great.