The theory is speculative. Now, speculative theories can be true, but there is no evidence to suggest that Jesus was in fact let down off the cross by anyone.

You may not be able to access this, but here is a link to a paper that reviews medical theories about crucifixion. The paper particularly concludes that traumatic shock would be a contributory or conclusive cause of death.

The modern discovery of antibiotics doesn't occur until the 20th century, so no, there was no possibility of antibiotics in that sense. There could have been some medical treatment in your hypothetical but it is unlikely.

Crucifixion was, by all accounts, an agonising way to die. 50% chance of survival for a soldier after 3 days would, I suspect, be incredibly optimistic. Crucifixion was, after all, designed to slowly and effectively kill you.

The strongest classical study of the topic is Martin Hengel's Crucifixion in the Ancient World, to which I would refer you for further reading.

I'm basing this off of Dr. H Somers/Koendad Ernst's speculation that he was let down after 3 days in a typical Roman sense of humor (We'll show you 'rebuilt after 3 days'), showed up and talked with apostles, then went off to Arabia and angrily delivered Revelations as a way to cope with the humiliation.

Could revelations have been his deathbed testimony as he died of infections? Would he have been likely to recuperate with contemporary medicine?