Is the trial of the grave and the questioning of the two angels only for those who are buried, or does it include every deceased person, whether he is buried or eaten by wild animals?

Firstly: 

The trial of the grave, and the questioning of the person in
his grave, about his Lord, about his religion and about his Prophet, is a
trial that is undergone by every accountable person in his grave. Whoever
Allah makes steadfast and he answers will be saved and will be granted bliss
in his grave, but whoever is tested and does not answer will be doomed. This
is indicated by shar‘i evidence. 

See the answers to questions no. 10403,
21713 and 72400 

Secondly: 

Every accountable person who undergoes this questioning will
undergo it after his death, no matter what his situation, whether he is
buried in a graveyard or not, whether all of his body or part of it is
buried, whether he died on his bed, or drowned in the sea, or was burned to
death, or cut into pieces, or eaten by wild animals, no matter where and how
he died. 

An-Nawawi (may Allah have mercy on him) said: Our companions
(i.e., the Shaafa‘is) said: 

The questioning of the two angels and the torment of the
grave cannot be ruled out because of the body of the deceased being
scattered, as we usually see, or by his being eaten by wild animals or fish
in the sea, and the like. Just as Allah, may He be exalted, will bring him
back to life for the resurrection, and He, may He be glorified and exalted,
is able to do that, in like manner He is able to bring part of him back to
life, or several parts, even if he has been eaten by wild animals or fish.

End quote from Sharh Saheeh Muslim (17/201) 

As-Suyooti (may Allah have mercy on him) said in his poem
Abyaat at-Tathbeet (83): 

The unburied person and the crucified person are questioned,
although the living are not able to see that. 

The commentator on the poem said: 

The questioning is something that will happen to everyone who
dies, no matter how he died, whether it was by knocked to the ground or by
drowning or by being thrown into the fire, or being eaten by wild animals or
birds, because of the general meaning of the evidence that speaks of this
questioning… end quote. 

Shaykh Ibn Baaz (may Allah have mercy on him) said: 

The grave is either one of the gardens of Paradise or one of
the ditches of Hell, and punishment or bliss happen to both the soul and
body in the grave… and also in the hereafter. The delights of Paradise
affect both the soul and the body, and the torments of Hell affect both the
soul and the body. 

If a person dies by drowning or by burning, or by being
devoured by wild animals, the soul experiences its share of torment or
bliss, and the one whose body is in the sea or was burned or is in the
bellies of wild animals will also have its share of that, in a manner that
is known to Allah, may He be glorified and exalted.

End quote from Fataawa Noor ‘ala ad-Darb (4/304) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

Is this – namely the trial of the grave – something that is
only for the one who is buried in a grave, because of the words “When the
deceased is buried”, or does it apply to everyone who dies? 

Answer: this is not only for the one who is buried in a
grave; rather it includes everyone who dies. The words of the Prophet
(blessings and peace of Allah be upon him), “When the deceased is buried…”
are based on what is usually the case. 

Based on that, if a person is knocked to the ground [and
remains unburied] or thrown into the sea and dies there, then the two angels
will come to him and test him. 

End quote from Sharh al-‘Aqeedah as-Safaareeniyyah
(1/433) 

So whether the deceased is buried in the proper manner or
not, the two angels will come to him and will test him in his grave, and
they will ask him in a manner that is known to Allah, may He be exalted.

And Allah knows best.