7. Is it forbidden for Non Muslims to enter military service


No it is not. They can join the army to defend their country and if they do so they are exempted from paying the Jizyah. History reported many similar cases where non Muslims joined the Islamic army to defend the Islamic state they live in. We have mentioned previously some of these examples. It was reported that the Islamic army led by Salah Eldin to defend Jerusalem against the crusades had many Christian soldiers in it who were actually defending their country against European occupation.
According to Shariah, it is not essential for any non-Muslim to render military service in an Islamic state because Jihad (military service) is only an obligatory religious duty compulsory only on Muslims.
In lieu of the military service, the non-Muslims will have to pay the Jizyah for their protection by the Islamic state but Muslim jurists have not stopped the non-Muslims from taking part in fighting voluntarily along with the Muslims in the interest of their state provided the authorities are convinced that they would not betray them in the battle field while fighting against their co-religionists if they happened to be the enemy. If they fight with Muslims they will not have to pay the Jizyah tax.