(1) Can a husband and wife pray in congregation? If yes, then how should they stand? If there is no possibility for a person to pray in congregation, then will he get the reward since he has the intention?
(2) In congregational prayers, is it forbidden for a single person to stand in one row? Should he ask someone in the row ahead of him to come back or should he wait for another person to come and join him. How long should he wait?
(3) Can two non-Mahram relatives (of the opposite sex) pray in congregation?
(4) What are the conditions under which one should perform Sajdah-i-Sahaw (prostration of oblivion). If you ask a common religious cleric, he would enlist numerous possibilities and a plethora of instructions which are hard to remember. One of them is that if you sit down for Tashahhud and then standing midway you realize that you are making a mistake, you should not sit but complete another Rak‘at. In prayers with four Rak‘āt if you have prayed an extra Rak‘at (the 5th one), then you should also complete another Rak‘at (the 6th one). The additional two would be counted as Nafal.
(1) Yes a husband and wife can pray in congregation. The wife should stand a little behind the husband.
As a principle, a person gets the reward of his intention if due to certain circumstances he is unable to accomplish what is required of him. Consequently, if a person has the intention to pray in congregation, and in his given circumstances he is unable to, he will get the reward of praying in congregation.
(2) A person should not stand alone in a row. He should gently touch a person in front of him to signal him to join him in the previous row. He should not wait for any other person. However, if the person in front of him is not aware of this procedure -- as is generally the case these days -- and asking him to join might create a problem, he has a legitimate excuse to join the congregational prayers while standing alone in the last row.
(3) Yes they can. However, in this case they should not stand side by side. The man should stand in front.
(4) In this regard, the Sunnah of the Prophet (sws) is that whenever a person forgets something in his prayers, he should try to rectify the mistake if it is possible and do Sajdah-i-Sahaw. If it is not possible to rectify the mistake, he should only do Sajdah-i-Sahaw. The errors and instances which need rectification and the actual method of this rectification have been left to his common sense. In all spheres where the Sharī‘ah has left matters to common sense, a person try to develop the habit of using his common sense.
Consequently, in your referred examples also, the rulings are based on common sense since the Sharī‘ah is silent on this particular issue.