Is it permissible to delay the regular Sunnah prayers until after the 'Ishā' prayer, and is it permissible to combine them and perform them separately (two rak'ahs at a time) at this time?

The regular Sunnah prayers are twelve rak'ahs. The time for the pre-Fardh Sunnah prayers begins with the entry of the time for the obligatory prayer, and the time for the post-Fardh Sunnah prayers begins after the completion of the obligatory prayer and extends until the end of the prayer time. One should not miss the regular Sunnah prayers except for an excuse. If someone misses them due to an excuse, such as sleep or forgetfulness, it is sunnah for him to make them up at any time he wishes, and this is indicated by the action of the Prophet (peace be upon him). However, if someone misses them without an excuse, it is not legislated for him to make them up, based on the saying of the Prophet (peace be upon him): "Whoever performs an action which is not in accordance with our matter, it will be rejected."