Angels,
On point 2: you are probably far from military aviation. Such episodes as entering each other's airspace happened tens (if not hundreds) of times between the US and Russia in the years of the "cold war". If it were for a short time (absolute majority of the cases), nothing happened - just follow and watch. The intruder, having seeing a "counter-part", usually gets out.
Also, there is a de-facto "protocol" on how to behave in such situations. In brief, if there is no response on the radio, approach, establish a visual contact and give a sign ordering to get out. If the order is not obeyed, engage a gun and shoot along the route showing that you are serious. It is also a well-known practice to escort the invader to the airbase. Despegue addressed this issue between Greece and Turkey a few post above in a quite comprehensive way.
As for point 3, you are advised to study the geography and variety of the armed groups in Syria. Besides ISIS (or ISIL) there is, e.g. a so-called "Nusra front" who is a local Al-Qaeda branch. Nusra front is a sort of a buffer between the anti-Assad rebels in the north-west of Syria and ISIS, which is east of Aleppo. Thus, those US backed rebels (who are hardly distinguishable from other terrorists) are not fighting ISIS, they are simply not in touch. The main local force who is fighting ISIS (as well as the Turkish forces) are the kurds in the north and north-east part of the country.
Regarding the bases in Syria, why not? The US have several bases in Turkey, the UK has a base in Cyprus, that's life...