Prophet Muhammad (PBUH) was gifted with the gift of salah at the age of 52. So how did he and all the other Prophets pray before Isra O Miraj? Did it include sujood or was it different? What was it like? How was it performed?
The Prophet Muhammad(saw) was gifted with salah as a mandatory at the age of 52.
There is evidence that the Prophets before and the Prophet Muhammad(saw) would pray before Israa’ wa mi’raj happened. In Surah Ali Imran, ayah 43 [3:43], Allah says to Maryam, O Mary, be devoutly obedient to your Lord and prostrate and bow with those who bow [in prayer].”
يَا مَرْيَمُ اقْنُتِي لِرَبِّكِ وَاسْجُدِي وَارْكَعِي مَعَ الرَّاكِعِينَ
We know this because from different ahadith as well. When Ali(ra) became Muslim at such a young age he begged the Prophet Muhammad(saw) not to tell Abu Talib. The way that Abu Talib found out Ali(ra) was Muslim was because he saw him praying with the Prophet Muhammad(saw). The way that they used to pray prior to the mandation of Salah, was slightly different. They used to only pray two rakats. It included both Sujood and Rukoo’.
When you mentioned Allah SWT giving his first command to Prophet Muhammad (PBUH) (Ya ayuhal mudhatir, Kum, Fa anzir, Wa rabaka fa kabir), you said that Allah SWT "told" Prophet Muhammad (PBUH). How did Allah SWT give/tell Prophet Muhammad (PBUH) that command? Was it through Angel Jibreel? Did Allah SWT always communicate with Prophet Muhammad (PBUH) through Angel Jibreel?
Allah communicated with the Prophet Muhammad(saw) in multiple ways, similarly to how he communicated with other Prophets in multiple ways. We see from the ahadith that the most common way of wahy, divine revelation was through the Angel Jibril(as). However, Allah would also inspire things to the Prophet Muhammad(saw) through a strong thought or feeling, he would inspire it in a dream, talk to him while hidden behind a barrier.
You also mentioned that Prophet Muhammad (PBUH) prayed every time he had a problem. How did Prophet Muhammad pray when he had a problem? Is there a particular way to perform that salah? Is it 2 rakats or is it just how much ever a person feels like they need to pray?
There are many ahadith that talk about how the Prophet Muhammad(saw) would stay up all night in salah. The narrations collectively entail that the Prophet Muhammad(saw) would pray in increments of 2 rakats, after every two rakats he would tasleem.
Is it permissible to recite duas/supplications in another language during salah?
It is permissible to recite duas/supplications in a language other than Arabic during voluntary salah not during obligatory salah.
If you are not allowed to recite Quran in Sujood, does that include Quranic duas as well?
You are not allowed to recite an entire surah in sujood, however you are allowed to recite quranic duas in sujood.
The opening supplication that I recite for salah is, “Subhanaka Allahuma wa bihamdika wa tabarakas muka wa talaa jaduka wa laailaha gairukah”, am I saying the wrong opening supplication?
This isn’t a wrong supplication, the difference in opening supplications and its variety is just that there are multiple narrations and multiple options for opening supplications.
Is the Tashahud a conversation between Allah and the Prophet Muhammad(saw)?
There is no sound evidence that states that the Tashahud is a conversation between the Prophet Muhammad(saw) and Allah.
Does the niyyah(intention) have to be a specific duaa or recited in Arabic?
Making intention to make salah does not have to be in arabic nor does it have to be a specific duaa. By getting ready, and standing up for salah it can be considered as intentions. Or simply saying, “I intend to pray such and such salah”
In the extra saying of the duaa سبوح قدوس رب الملائكة والروح during salah, why is there a dumma’ on the rabb if rabb is mudhaf?
A mudhaf can take fatha, kesra, or dumma. The mudaf ilayhi, has to have kesra and be jarr.