Is it valid to pray the two rak'ahs of Sunnah before 'Ishā' and then a single rak'ah of Witr after 'Ishā', and is it valid to omit Witr and suffice with the two rak'ahs of Sunnah when one is delayed in praying 'Ishā'?

The supererogatory prayers (Sunan) performed before or after obligatory prayers are of two types:

First: Regular Sunan (Sunan Ratibah) which the Prophet, peace and blessings be upon him, consistently performed. These are: two rak'ahs before Fajr, two rak'ahs before Dhuhr and two rak'ahs after it, two rak'ahs after Maghrib, and two rak'ahs after Isha.

Second: Non-regular Sunan (Sunan Ghayr Ratibah), which are all others not mentioned above.

The regular Sunnah for Isha is two rak'ahs after it. It is also desirable to pray two rak'ahs before it, based on the Prophet's saying, peace and blessings be upon him: "Between every two adhans (i.e., adhan and iqamah) there is a prayer for whoever wishes to pray it."

As for Witr, it is an independent Sunnah. Its time begins after the completion of the Isha prayer and ends at the break of dawn. The minimum number of rak'ahs for it, according to the soundest opinion, is one rak'ah, based on the Prophet's saying, peace and blessings be upon him: "The night prayer is two by two, and if one of you fears the coming of dawn, he should pray one rak'ah to make the prayers he has already prayed odd-numbered." The maximum number is eleven rak'ahs, and some say thirteen rak'ahs.

It is preferable to pray Witr with three rak'ahs to avoid the disagreement of those who hold that the minimum number of Witr rak'ahs is three.