I live and work in Canada. It is getting very difficult for me to be punctual in Salat that leaves unrest in my heart all the time. I have heard that in this kind of difficult situation I can perform Zuhr and Asr and Maghrib and Isha Salat at a time. Meaning from the start of Zuhr time to end of Asr time I can perform both the Salats. Same for Maghrib and Isha. Please correct me or verify it? Secondly can I perform all Salats at night after getting back to home?
Preferably one has to read each of his daily prayers on time and during their respective times. As long as you can, with a reasonable effort, read your prayer within its specific time, you should do that, even if it means that you have to read it later in its time (i.e. not on time).
However in the situation where this creates much difficulty, one may combine his Zuhr and Asr and combine his Maghrib and Isha, as long as this does not become a routine even at the time of comfort. This is inline with the concept that the Qur'an has referred to a lot, that is, God does not want to put us in difficulty by his rules. The verse that allows shortening prayers during travel (4:101) and the verse that implicitly indicates flexibility in the timing of prayers (4:103) are due to the same concept.
It is based on this that we see the same flexibility reported from the Prophet (pbuh) where he would join the Zuhr and Asr prayers as well as Maghrib and Isha prayers at the time of hardship.
The method of combing the two prayers that you mentioned is correct.
It is not permissible to consciously leave all your prayers till evening because this means your Zuhr and Asr prayers will be due. If we want to do only what is obligatory, we can perform our prayers in a very short time. This, with the above mentioned flexibility of the time of the prayers should allow any one to find a space and a brief time to perform their prayers within acceptable time frames.