There are also a lot of unwritten rules. Invasion is not one thing. It can happen in a milion ways. Each one can have its own level of severity and justify its own set of retaliations. War can be fucking complex. A good example is that no one gives much of a shit about planes bombing ISIS even if it is technically illegal.

You are right though, going against Assad is very likely to invoke a hefty response. If Turkey does that it will likely go out of its way to do things in a tricky way that makes a response difficult. I would love to see Russia fuck them royally but while I understand many of Russia's concerns their POV might be different as the ones who actually have to make the decision to commit to action.

Iran is unlikely to do shit on its own when it comes to total war however if Russia goes full scale war then you can expect Iran to be somewhat likely to also bring its full forces to bear. The state Iraq is in means Iran could probably get away with just waltzing its whole army across. Iraq isn't happy with Turkish incursions either.

The domestic situation in Turkey can be exploited as well. Erdogan isn't that popular with an uprising against him well justified. You also have unstable elements internally that can challenge the state such as the Kurdish situation.

Russia is also going to be a big Devil's advocate to Greece as well to fuck things up in the west for Turkey since Greece is fucked with the EU anyway. It is in a situation where its alignment could be shifted and has grievances against Turkey. Armenia as well and you can guess where Russian troops will come from.

The problem is that the potential threat of that might not be enough to deter Turkey. All this rationalisation is pointless when discussing insane leaders like Erdogan whose modus operandi is self deception.

It doesn't help that Turkey also has a defence guarantee from NATO. If Turkey sends 100000 troops marching into Syria and they get tactically nuked into dust then tough luck. But if Russia might invade Turkey or take territory from it then NATO might step in with the idea of that being a limited war. Russia will be allowed to defend Syria. NATO will be allowed to defend Turkey. But they wont be allowed to attack either state of the opposing states they are supporting their state against directly. Those are the default and standing rules of engagement at present as the situation prescribes.

The bases in Turkey are locked in to this. Turkey has shields in many of its bases. If to defend Syria Russia has to hit Turkish bases in Turkish territory to for example stop Turkey projecting air power over Syria (Saudi involvement will make the air battle much harder for Russia) keeping in mind Turkey will also be hitting Syrian air bases then Russia may have to strike bases containing foreign NATO assets and personelle thus destroying them and greatly involving NATO as a whole in the affair. NATO is also a bit stuck there because it is there as a large multinational force committed to fighting ISIS. If Turkey sends in ground forces then Russia is almost certainly going to want to hit those forces with air. If Russia starts destroying the invading army with air Turkey is going to want to event things out with its own air engagement. There is going to be a good chance that to deal with that Russia will need to take out Turkish airbases and wham.

It is very hard for Turkey to not suck NATO into that in someway that protects Turkey from that ultimate national sacrifice if it happens. Losing troops is one thing. They are expendable at the end of the day. They care but nothing eternally damages like losing territory or being completely invaded or destroyed does. Although given Turkish pride we could be fucked in such a scenario as they might have problems with things such as accepting defeat.