Can masah of the head be performed with hair oil applied over it?

If the oil is a barrier then you should make sure the water reaches the scalp and it will be correct.

I am a student of knowledge and I have thirst and love for knowledge but one day suddenly my thirst for knowledge have been lost and I lost interest at learning anymore
I'm confused about what is the true purpose of knowledge, in my mind knowledgeable and ignorant person is the same in their own sense(its just my thoughts) and in my mind it's fine to be ignorant and I don't know what's happening. How can you convince me to seek knowledge, what is the true purpose of knowledge

I would highly recommend you to read this book especially the introduction.

https://www.islamic-college.ac.uk/shop/desire-aspirant-teacher-student-pbk/

Because of the corona virus my neighbours are scared to go outside and to buy groceries. Is it permissible to go buy groceries for them? For example to touch and buy non halal meat. I really want to help them but i dont know if it is permissible

It is permissible to buy them non halal meat if they are non Muslims but it is not permissible to buy them pork.

A quick question regarding mouthwash (for bad breath, not water) while fasting, if I use it and make sure to spit it out at least 3-4 times and not swallowing anything during this process, but afterwards when you swallow the saliva I can feel a little bit of taste/liquid, is this fine?

Sayyed Sistani says:

"Washing the teeth does not invalidate the fast as long as the person does not swallow the saliva that has mixed with the mouthwash or toothpaste. However, the lingering flavour or taste of the paste that mixes with the saliva does not affect the fasting."

How do I determine if the saliva I swallow afterwards is just the taste mixed with it or if its a bit of the moutwash mixed which is not allowed? Or can I be sure that when spitting it out at least 3 times it is fine whatever taste I feel?

Is it permissible to eat meat from a restaurant owned by Arab Christians if they are advertising their meat as halal? How can one have certainty? If a Muslim tells you their meat is halal, you should take their word. Does the same rule apply to non Muslims?

You could investigate to see their halal certification.

On Ayatollah Sistani's website he sometimes writes: "This ruling is based on obligatory precaution, in this ruling, you may refer to the next learned Mujtahid." Does this last line mean that we can use a different Mujtahids ruling?

Yes.

I am reading the English translation of the Quran and a verse troubles me, as I am unsure of its meaning. Could you explain verse 26 of Surah Baqara, where Allah uses the example of a gnat?

“God does not shy away from making an example of a gnat, or something above it. As for those who believe, they know that it is the Truth from their Lord. But as for those who disbelieve, they say, "What did God intend by this example?" He leads astray many thereby, and He guides many thereby; but He misleads thereby only the evildoers.”

Allah says in another ayat:


يَا أَيُّهَا النَّاسُ ضُرِبَ مَثَلٌ فَاسْتَمِعُوا لَهُ ۚ إِنَّ الَّذِينَ تَدْعُونَ مِن دُونِ اللَّهِ لَن يَخْلُقُوا ذُبَابًا وَلَوِ اجْتَمَعُوا لَهُ ۖ وَإِن يَسْلُبْهُمُ الذُّبَابُ شَيْئًا لَّا يَسْتَنقِذُوهُ مِنْهُ ۚ ضَعُفَ الطَّالِبُ وَالْمَطْلُوبُ - 22:73


“O people, an example is presented, so listen to it. Indeed, those you invoke besides Allah will never create [as much as] a fly, even if they gathered together for that purpose. And if the fly should steal away from them a [tiny] thing, they could not recover it from him. Weak are the pursuer and pursued.”

So He is saying that the disbelievers idols cannot create even something as small as a fly. Similar in this ayat you mentioned Allah is saying we can even look at something as small as a gnat and how Allah has created it with life and all the necessary parts it needs. This is one interpretation.

We do not have to go look at something very large as an elephant to see the complexity in Allah creations we can see the complexity in something very small like a gnat and this makes us say Subhanallah.

I have always wanted to pray salah and recite Quran but I just pray for 2 days then I leave for long time. I even to ask Allah to give me taufeeq to become regular in namaz and recite Quran but nothing happens. Please advise

This requires several things

1) Learning the benefits of making salat which can be done through reading upon the subject or listening to majalis on the topic.
2) Making intention and asking Allah constantly for His help
3) Disciplining your nafs to get in the habit of prayer. You should set an alarm on your mobile to help you with this.

These are some of the ways we can improve.

Just curious to know is there any negative energy surrounding us? And can anyone do magic and make the person weak. Is it possible? And any dua to be protected from it?

We do know that magic is real, because the Quran mentions the magicians of Pharaoh. Also when we read surah Falaq 113 in the Quran we are seeking refuge from magic. The last two surah from quran are effective (for protection)

I live in an area where the closest Shi’a masjid is too far away to travel during work on Jummah. What should I do? Should I pray regular Dhuhr at work only or pray Dhuhr at work then try to go to a Sunni Jummah service.

You should pray Zuhr at work. If you go to the jumah service of a non shia then you would still need to make salat al zuhr.