What is the ruling on entering Mecca without being in a state of ihram, then entering into ihram after entry and slaughtering an animal to avoid being prevented from entering Mecca during Hajj?

It is not permissible to pass the Miqat without assuming Ihram for anyone intending to perform the rituals. The Miqat for those residing within its boundaries is their own city or country. Ihram is the intention to commence the rituals at the Miqat.

Whoever fears being prevented from entering Mecca and wearing Ihram clothes must offer a fidyah (expiation) (a sacrifice of a sheep, or fasting for three days, or feeding six poor people). The clothes must be removed immediately once the danger has passed. This applies if the Hajj is obligatory, or if one is already in a state of Ihram, or if one is assisting someone performing an obligatory Hajj.

If this is not the case, it is advised to stipulate at the time of assuming Ihram that one will exit the state of Ihram if prevented from completing the rituals. If permission is granted to proceed, then all is well. Otherwise, one exits the state of Ihram, and nothing is due from them.