(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
(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