During Tiruvaradhanam, even when the number of available fruits is less, say 1 or 2 pieces only, can they be offered to Perumal? What to do with the fruits that were offered to Perumal?

If it is banana fruit, two must be offered. If a fruit is unfit for our consumption, it is not offered to Perumal. One or two apples and other similar fruits can be offered.
Notes:
Leftover fruits that were offered earlier must not be re-offered to Perumal. Fruits left over after we partake or fruits that are bitten by rats, must not be offered. Without any such defects, even half a fruit can be offered as pieces, and we can later partake them.