Much of the evidence presented on Sinaiticus.Net is proof of someone tampering with the manuscript (colouring and ageing it) and observations that refute the purported age of the manuscript, and refute the story told by Constantin Tischendorf.

Do we have conclusive proof
that Contantine Simonides was involved in the production of the manuscript that became Codex Siniaticus?

In his vlog, "The Sinaiticus Smoking Gun?", David Daniels points out three strong pieces of evidence that have convinced him that Constantine Simonides was involved in creating the manuscript between 1839 and 1841 at Mount Athos. 

1. Spyridon Paulou Lampros published a two volume catalog of the Greek manuscripts on Mount Athos in 1895 and 1890.  This catalog shows that Constantine Simonides, his uncle Benedict, and the monk Kallinikos Monachos were all present on Mt. Athos,
at Panteleimon monastery.  Simonides and Kallinikos are even shown to have been working on the same project at the same time.

The catalog can be found at these links:
Volume 1
Volume 2

This catalog also confirms the very existence of Kallinikos, who wrote a letter in 1862 from Alexandria, Egypt, declaring that Simonides himself was the scribe of the manuscript which was later taken by Tischendorf from the Sinai monestary. Kallinikos not only accused Tischendorf of stealing part of the manuscript ("perusing and reperusing it frequently, abstracted secretly a small portion of it"), but also of applying lemon juice to the remainder of the manuscript to cause it to look older ("to weaken the freshness of the letters").

2. The text of The Shepherd of Hermas in Codex Sinaiticus is almost identical to the text that Simonides published. 

It turns out that the text of the Epistle of Barnabas in Sinaiticus is practically identical to the text published by Simonides, when corrected by the footnotes he included from the text published by Gaspareos. 

The Barnabas text in Sinaiticus also contains little correction notes all through the text.  When the main text is corrected with these notes it matches the main text that Simonides published! 

So it is clear that the two main texts are linked by a set of published corrections!  Simonides possessed the main text AND the corrections to create the Barnabas text of Sinaiticus.

3. Constantine Simonides claimed that he had written some identifying notes into the text.  These notes should prove that he penned the codex.  One of these notes (acrostics) was supposed to be written in the margin of the text at Genesis 24.  Unfortunately, the margin, as well as other pieces of this page are MISSING.  Could it be that these pieces were destroyed on purpose to obliterate the identifying marks?