I've written recently a post on a very similar question.

But might as well repeat some info here on João Vaz Corte-Real and others. I am now mostly citing from the appendix in the book Foundations of the Portuguese Empire 1415-1580 by Diffie and Winius, which I have at hand, but I would better recommend reading Samuel Eliot Morison's book Portuguese voyages to America in the fifteenth century (available to be borrowed on archive.org) which goes into more details and deeper analysis and information.

To sum up the part abouz João Vaz Corte-Real voyage. The main and pretty much only source claiming this alleged voyage happened in 1470s is

Caspar Frutuoso who wrote his Saudades da terra a century later, about 1570-1580.

So a much later author. The modern critics also point out the following:

Because of the lack of other evidence to support Frutuoso, the question of his reliability is essential. Morison enumerates many errors and confusions in his works. So does Duarte Leite, who sets most rigorous standards for documentation. For example, Morison notes that Frutuoso credits Joao Vaz with discovery of Terceira, Cape Verde, and Fogo, though all were discovered before, and with the discovery of Brazil which occurred later. Furthermore, he supposedly won great battles from Castile, "as captain of great fleets," capturing from the Castilians "the best prizes that ever in this realm of Portugal were taken from them." But none of this appears in any known document, nor was it recorded by any historian before or contemporary with Frutuoso.

Frutuoso, says Duarte Leite, "fell into numerous confusions, gave wrong dates, and committed errors of fact. In spite of his diligence in recounting the discovery of Santa Maria, what he asserts is unacceptable in totum, and he also errs in what the says of the Island of Sao Miguel."Leite describes Frutuoso's biography of Joao Vaz as "full of nonsensical legends and . . . various errors"

Basically, Frutuoso's work is so full of errors, inconsistencies and irregularities, that it simply can not be accepted as relevant and truthful source of information for any bold claims. Furthermore, Corte-Real family was a relatively important family in Azores and later exploration of Newfoundland, and as a consequence we have several official documents, grants, donatories, as well as biographies and texts about both João Vaz and his sons Gaspar and Miguel, none of which even hints at any sort of voyage of discovery earlier then the one of his sons in late 1490s.

To proceed with the other mentioned names. Didrik Pining's is also very briefly mentioned in the same chapter with the following note:

The efforts to connect the alleged discovery of Newfoundland by Joao Vaz Corte-Real with the known voyage of two Danish seamen, Pining and Pothorst, and with a trip by Johannes Skolp (Scolvus) in 1476, rest on no substantial information, though they have been widely discussed by numerous historians. Nor does anything indicate that the Pining-Pothorst or the Scolvus sailings went beyond Greenland, a well-known area in the fifteenth century.

Case of Afonso Sanchez is also briefly analyzed:

Las Casas, who recorded all the gossip he heard, tells a tale of a reported informant of Columbus, of unknown nationality but sometimes called Alfonso Sanches, who was said to have discovered land to the west before Columbus and to have told Columbus about it. Columbus does not refer to this anywhere. Ferdinand Columbus perhaps confuses this alleged voyage of the "Anonymous Pilot" with the real voyage of Vincente Dias to Cape Verde in 1444. Dias "saw" an "island" which he searched for in vain in three or four voyages financed by Luca de Cassano

Morison in the other book (Portuguese voyages) also points out that even in Las Casas story the sailors didn't actually land in the area they claim to have seen, and further shows numerous others cases of sailors spotting islands in the distance in various places of Atlantic where we now know there aren't any. So even if this recorded information - which was at that point only second (or third) hand rumor - was completely honest and in good faith, it could just be a mistake or erroneous belief by the spotter. And certainly not a proof of event happening.

In the end I highly recommend the sources I listed above, they are quite informative and thorough to check out, and deal with all the cases you mention, of which none has anything remotely firm to back them up, and even the sources from which the claims stem are vague, undefined and unsupported