In Heroes of the Fallen Lands many of the core feats have been streamlined, some paragon feats have become available to all levels.
Before that feats were tiered by level: heroic, paragon, epic.
Can a character that was made before the Heroes of the Fallen Lands publication use Heroes of the Fallen Lands feats?
Yes
D&D Essentials is part of D&D 4th edition, and makes no differentiation as to source of feats. As essentials is entirely compatible, any character may take feats, utility powers (if appropriate), and items from the essentials books.
Check out this question.
However, it is important to note that the inclusion of any book or suppliment in an RPG is always subject to DM discretion, and so your DM may choose to disallow the book for any reason.
As a point of order it is not appropriate to give characters the new and improved expertise feats for free. Looking at the math here, DMs can draw their own conclusions about versatile expertise being necessary. The additional abilities granted by the new expertise feats, like ignoring cover, scaling damage, free actions to reload crossbows, and others, are beyond the scope of a simple math fix.
Yes,
There is no rule against it and the online character builder allows it.
Unless your DM is against it there should be no problem.
Some essentials feats are strictly better than previous feats and you should upgrade them when you have the chance. (Remember each level you gain you are allowed to trade out one item such as a feat and replace it with another).
