At this point the "miracles" you reference are anectdotal. If in fact there were a literal physcial presence, then these miracles would be present at every communion. They're not. If they still occur today, then the only way to make a determination that they are the actual blood and flesh of Christ is to do a DNA test everytime the miracle occurred. If the DNA matched in all cases, then I think your case would be much stronger.
Right now, however, since the elements do not change physically, then the evidence suggests that there really is no physical change. It may well change literally on a spiritual level, but on a physical level there really is no scientific evidence that suggests that it really happens.
It is an exercise in Faith and not an exercise in digestion.
All miracles are anecdotal by definition.
Because he doesn't need to demonstrate it.
If signs and wonders are demanded, God often says, Tough.
But sometimes he gives them as presents.