Is it permissible to go to Makkah without ihram, with the intention of entering into ihram and performing Umrah from Makkah, if a permit is available, and otherwise not?

If you intend to enter Makkah without a firm intention to perform Umrah, there is no harm in entering without ihram; because ihram is obligatory for one who passes the Miqat with the intention of performing Umrah.

If you obtain a permit, go out to Al-Taneem or the nearest point outside the Haram boundary and assume ihram for Umrah from there. This is because whoever is in Makkah and wishes to perform Umrah must assume ihram from the nearest point outside the Haram boundary, and it is not permissible for him to assume ihram for it from within the Haram. The majority of scholars hold that all directions outside the Haram boundary are equally valid, and it is not exclusive to Al-Taneem.