face to the grave of prophet Muhammad (PBUH) say Salam directly to prophet
Not only the prophet's(pbuh) but to say the following in your inner self when ever you come across any grave(Muslim) is highly recommended.
Peace be upon the inhabitants of the graves, believers and Muslims. May Allaah have mercy upon those who have gone ahead of us and those who come later on, and verily we will, Insha’Allaah, join you. (Thirmidi, Janaaiz 58-59 narrated by Ibn-i Abbas)
The above Hadith which I quoted is not to be mistaken. Our intention is to be just wishing peace on the people in the graves including Prophet(pbuh), but not wishing them as though you wish a person you just met saying Assalamu Alaikum.
Prophet is passed away and he is no longer alive to hear us or reply our Salam.
Surely you do not make the dead to hear, and you do not make the deaf to hear the call when they go back retreating. (Qur'an 27:80)
So, Qur'an literally says so. So we must not be having any doubt about it.
We should only ask Allah. Asking something from prophet is Shirk.
Yes. Because you yourself said,
You (Alone) we worship, and you (Alone) we ask for help(Qur'an 1:5)
(Note: In terms of Ghaib(unseen) we ask only to Allah , I'm not talking about asking your friend to get a cup of coffee)
We should face only towards Qibla. Facing towards the grave of prophet is Haram.
You need not face only towards Qibla and you should not face only towards the grave of Prophet(pbuh). As I said, intention matters, not the direction.
they ask him to ask Allah something on behalf of them, because he has more reputation near Allah.
The concept of Prophet(pbuh) being given the power to intercede on our behalf is mistakenly deduced to the fore mentioned belief. It is on the day of Judgement, when Prophet (pbuh) because he has more reputation near Allah will be able to intercede as Allah permits. And Prophet(pbuh) will start interceding about those who are closer to him on that day. So, asking Prophet(pbuh) directly doesn't make you get closer to him, but doing so keeps you far away from him. To get closer, you need to ask Allah to send blessings on him(Durood). That is the prescribed way.
You don't require a medium today to ask forgiveness, because, the creator says
And indeed We have created man, and We know what his ownself whispers to him. And We are nearer to him than his jugular vein. (Qur'an 50:16)
And seek Forgiveness of Allah. Verily, Allah is Oft-Forgiving, Most-Merciful. (Qur'an 73:20)
May the creator guide us all.