I know that tohfatulawam contains some pages with a direction to the questions you have in mind. I wanted to know what are the etiquettes of acquiring an answer, is there something like a dua or ism e azam we have to read before opening the book for the answer? I will appreciate any kinds of help! Also may I ask some effective duas and amal to ask for the impossible? I know nothing in front of Allah swt is impossible and I don’t want to lose hope either, maybe a dua or amal would work.

As far as Istekhara is concerned, you may recite the dua of Ishtekhara and look it. Sometimes people do Istekhara with Tasbeeh or with Holy Quran but what you are asking about doing Istekhara from Tohfatulawam, there is no such Istekhara mentioned in traditions of Aimmah (a.s) You may search on google for traditions and method of doing Istekhara according to Aimmah (a.s) you will find the way and dua also for that.

Another things as you asked about some amal to make anything or something possible which seems impossible. In this concern I would say Allah (swt) has created this universe with Wisdom and Logic therefore we should not deal with force or stubbornness. We as a human are given this authority to strive for our life and rely on Allah. Sometimes for that very purpose we have to ask special assistance of Allah but on the top of that our action is the most important part of the task. So nothing is impossible in that sense since we don’t know the reality of that thing. And this is also a part of our system of this world that we should not be aware about the reality of that thing otherwise our life will become meaningless.
So please rely on Allah and do continue your efforts Allah will give you success and if you would get clear success make sure Allah will replace it with something which is better than that.