As it applies to your question, there are two basic flavors of Protestantism. One kind consists of groups that have some form of centralized organization who could at least be said to have an “official” stance on a given position. The other kind has no formal centralization (they believe each congregation—and individual—to be autonomous in terms of specific beliefs and practices) and, thus, no capability of having an “official” stance on any position.
Since the latter vary widely in their particular beliefs, it is impractical to seek out sources since individuals would choose whether or not to agree with them. You might find academics who consider themselves a part of these groups, but their statements are by no means representative of other similarly self-proclaimed denominational adherents.
Though the second kind has no hope of providing an answer, the first kind does have the capability of providing official statements of doctrine. The difficulty here is that if they do not, generally, have “official” statements or lists about things that they do not believe.
Here are a few major protestant denominations. For most of them, I could not find any information with searches or examining documents of core beliefs. The search term “relic” frequently turned up results, but I could find few references to “holy relics.” Most of the search results were only to documents containing the common English use of the word signifying “an object surviving from an earlier time.” If this silence says anything, it says that the holy relics of Roman Catholicism and Orthodoxy get far less mention than the common use of the word.
Lutheran (Missouri Synod): No mention of incorruptibility or relics
Anglican (& Episcopal)
- Core Beliefs: Thirty-Nine Articles of Religion
- Core Beliefs: (Episcopal USA) Core Beliefs and Doctrines
- The only things in this list described as “holy” are the Spirit, the Scriptures, Baptism, and Communion.
(from the Thirty Nine Articles of Religion)
The Romish Doctrine concerning Purgatory, Pardons, Worshipping and Adoration, as well of Images as of Relics, and also Invocation of Saints, is a fond thing, vainly invented, and grounded upon no warranty of Scripture, but rather repugnant to the Word of God.” - See more at: http://www.anglicancommunion.org/resources/acis/docs/thirty_nine_articles.cfm#sthash.y10sue2g.dpuf
Methodist (There is more than one group with “Methodist” in the name, but I think the United Methodist Church is the largest.)
Baptist (There are many types of Baptists, but the Southern Baptist Convention is a convocation to which many Baptist congregations subscribe.)
- Core Beliefs: Basic Beliefs
- “Holy Bible,” “Holy Spirit,” and “holy trusteeship”
- no mention of relic or incorruptibility
- The only mention of “incorruptibility” that I could find has to do with the post-resurrection body/world.
As a rule, Protestant churches either deny the sanctity of incorruptibility and relics, have no official position on the matter, or have no “official” positions whatsoever.
It might be possible to find treatments of the matter conducted by Protestants, but these would only be working from within a belief framework adhering to a particular protestant group’s beliefs (and almost certainly not uniformly) and nothing like an official position. How you would define credibility in this regard is somewhat subjective.
Probably the biggest serious objection that any disbelieving (in incorruptibility) protestant would make is that this belief has no basis in Scripture. The vast majority of protestant groups operate sola scriptura.