So is there any sin that Bahai's consider unpardonable?
Is it possible that all souls will eventually be saved?
Again you ask some great questions inquirer!
I think maybe in your question there is a background in the terms "unpardonable sin" and of course "being saved" also has a lot of implications for people.
When I was first introduced to Baha'is a long time ago I rarely heard the term "sin" used that much or "salvation"..these words are the words I used to hear in some churches I visited.
But anyway let's look at them in a Baha'i context.
Now here's an interesting quote and it contains some of the same words that are familiar to many Christians:
Abdul-Baha is quoting Baha'u'llah:
"My captivity is not My abasement: by My life, it is indeed a glory unto Me! But the abasement is the action of My friends who connect themselves with Us and follow the devil in their actions. Amongst them is he who taketh lust and turneth aside from what is commanded; and amongst them is he who followeth the truth in right guidance. As for those who commit sin and cling to the world they are assuredly not of the people of Baha."
~ Abdu'l-Baha, A Traveller's Narrative, p. 42
Here's a passage where Abdul-Baha uses the term "unpardonable sin"...
"...it is enjoined upon the father and mother, as a duty, to strive with all effort to train the daughter and the son, to nurse them from the breast of knowledge and to rear them in the bosom of sciences and arts. Should they neglect this matter, they shall be held responsible and worthy of reproach in the presence of the stern Lord.
"This is a sin unpardonable, for they have made that poor babe a wanderer in the Sahara of ignorance, unfortunate and tormented; to remain during a lifetime a captive of ignorance and pride, negligent and without discernment."
~ Abdu'l-Baha,
Baha'i World Faith p. 398
But I would say that the "unpardonable sin" for most Christians is probably the sin against the Holy Spirit.
So terms like sin, the devil and unpardonable sin are found.
But in the Baha'i context there is no original sin as it's commonly referred to..The following passage relates:
"But the mass of the Christians believe that, as Adam ate of the forbidden tree, He sinned in that He disobeyed, and that the disastrous consequences of this disobedience have been transmitted as a heritage and have remained among His descendants. Hence Adam became the cause of the death of humanity. This explanation is unreasonable and evidently wrong, for it means that all men, even the Prophets and the Messengers of God, without committing any sin or fault, but simply because they are the posterity of Adam, have become without reason guilty sinners, and until the day of the sacrifice of Christ were held captive in hell in painful torment. This is far from the justice of God. If Adam was a sinner, what is the sin of Abraham? What is the fault of Isaac, or of Joseph? Of what is Moses guilty?"
~ Abdu'l-Baha,
Some Answered Questions, p. 119
Let's look at salvation...
"O Thou Who art the Lord of Lords! I testify that Thou art the Lord of all creation, and the Educator of all beings, visible and invisible. I bear witness that Thy power hath encompassed the entire universe, and that the hosts of the earth can never dismay Thee, nor can the dominion of all peoples and nations deter Thee from executing Thy purpose. I confess that Thou hast no desire except the regeneration of the whole world, and the establishment of the unity of its peoples, and the salvation of all them that dwell therein."
~ Baha'u'llah,
Gleanings from the Writings of Baha'u'llah, p. 242
It does seem here that as revealed by Baha'u'llah, God's desire is
the salvation of all them that dwell therein." applies to everyone. Salvation is also connected to
the regeneration of the whole world and the establishment of the unity of its peoples..
Let's see how the term "devil" used in a Baha'i context:
"Regarding your question relative to the condition of those people who are described in the Gospel as being possessed of devils; this should be interpreted figuratively; devil or Satan is symbolic of evil and dark forces yielding to temptation."
(From a letter written on behalf of the Guardian to an individual believer, November 2, 1938: Spiritualism, Psychic Phenomena and Related Subjects, p. 4)
(Compilations, Lights of Guidance, p. 513)