No, the +X to either side cards, monster enhancers and so on
don't stay in effect while they are not on the table. However,
Sylph Preservation can be used at two different times, either after you play or after you discard a card. For those one-shots that apply a bonus until they are discarded, choose to use
Sylph Preservation when you discard them. For those that apply an immediate effect, like
Pollymorph Potion, use the immediate effect after playing the card of
Sylph Preservation.
The only problem here is that it says you must discard the cards, so I guess you can only get cards back if it's your combat, as the combat owner discards all the cards. It does allow you to actually get some neat cards from other players, though. Perhaps this is incorrect intent due to the use of "back into your hand", though.