I have a few things to add to your understanding of BTA's:
First, assuming you have a healthy specimen with adequate lighting, direct feeding is not entirely necessary. Feeding will cause an excess of nutrients in the animal causing it to grow much faster than without feedings. Many like to feed for the simple fact that it looks cool, and you are giving it human qualities that it likes being fed. Truth-be-told, it can't not feed. If a food particles touches its tentacles, it will attempt to eat it. That leads to the second point:
Second, no, you cannot overfeed an anemone. Once they take in food, then they "decide" (though there really is no thought process, they have no brain) whether to eat it or expel it. If there is too much food at once, it will expel some to allow it to digest the rest (think of having your mouth too full). BTA's are fully capable at self-regulating how much it can or cannot digest. It is, however, not advisable to give it such portions that cause it to expel any food. This is a net loss in energy, and can contribute to a decreased health of the animal.
Three, as 2Addicted touched on, silversides. The current state of knowledge on the affect of silversides on the health of anemones is that the food has gone bad, or essentially expired, perhaps to just being old, or from multiple freeze/thaw cycles during and after shipping. This allows bacteria to colonize the said fish and transfer harmful bacteria to the anemone. Sickness has not been attributed to quantity of food, but quality. I recommend only feeding raw, human grade seafood to anything in your tank - with the exception to my rule being PE mysis. Foods packed in water or juices seem to have a lower risk of bacterial contamination than those that are not - though I do not have any empirical evidence to back that claim up just yet.
Deflation is common, and natural. It is how they expel waste, and bring in fresh water to the body tissues. Some people have theirs deflate/inflate on daily cycles. You only need to become worried if it stays deflated or becomes deflated consistently or for long periods of time when your lights are on.
