An interesting, important and valid question. The short answer is that one can be a Seventh-day Adventist and not believe that EGW as a prophet of God. However, there are those who will immediately disagree with my statement. The problem arises due to misunderstandings of the so-called 28 Fundamental Beliefs and the so-called Baptismal Vow.
My position, as I have practiced it, is that the Baptismal Vows define the minimal requirement for entry into the SDA Church. In regard to this matter, the historic Vow, as it continues to exist today, says: "Do you believe [I believe] the biblical teaching of spiritual gifts and believe that the gift of prophecy is one of the identifying marks of the remnant church?" This statement requires that one believe that there is a valid "gift of prophecy" that will exist in the church of God at some unstated point in end time. It does NOT require a belief in any specific person. E.G. It does not require a belief in EGW as a manifestation of that gift. This set of Baptismal Vows consists of some 13 statement of belief.
In recent times an alternative see of Baptismal Vows has been produced that has been reduced to three (3) simple statements. This set of three does not contain any explicit statement on this issue, as is contained in the 13. My personal position is that this alternative set does not require a belief in EGW as a prophet. However, others challenge me on this and believe that it does contain an implied statement of a required belief in EGW as a prophet. My response to that is that if this is so, it is only the alternative set that requires it and the so-called thirteen does not.
In any case, the problem arises due to the so-called 28 Fundamental beliefs. This is a statement which is longer than that of the Baptismal Vow and it says, in part: "One of the gifts of the Holy Spirit is prophecy. This gift . . . was manifested in the ministry of Ellen G. White. . . . the Bible is the standard by which all teaching and experience must be tested." If this fundamental belief is a standard by which membership in the SDA Church should be measured, then a specific belief in her is required. But, tyhta is not how I understand the so-called 28. [NOTE: In its original formulation it was 27 fundamental beliefs. An additional one was added later to make 28.]
The editors of the 27, which was published in 1988, said: "We have not written this book to serve as a creed, a statement of beliefs set in theological concrete." IOW, it was not published, in my thinking in an attempt to define the standard by which one should be determined to be a member of the SDA denomination. Rather, it was published to put into print the most commonly held beliefs of SDA members. Certainly, most SDA members believe that EGW was a prophet, even if they disagree on exactly what that means.
So, in brief, the above is my personal understanding: The belief in EGW As a prophet is not required to be a member of the SDA denomination. Further, there is no clearly defined belief as to what it means to say that she is a prophet.