Here are the questions I have on these.
1. What is the denomination of the coins?
The first coin is either a 1/2 Bazaruco, a Bazaruco or 2 Bazarucos. In 1686, all these three denominations were issued. 1668 can be ruled out, as no coppers were minted in that year. You can determine the denomination by the diameter and weight of the coin:
1/2 Bazaruco: 14mm, 2.6 grams
Bazaruco: 18mm, 3.7/7.0 grams
2 Bazarucos: 24mm, 6.8/8.2 grams
The second coin is thougher to identify. It can't be a 1619 coin, and in 1691 no coppers were issued. Are you sure the last 1 is realy a 1?

Again, the options for the denomination are the 1/2 Bazaruco, the Bazaruco and the 2 Bazarucos, and the denomination can be determined by the diameter and weight of the coin. The coin could have been minted either during the regency of prince Peter, or during his reign as Peter II. During the regency the diameters and weights were:
1/2 Bazaruco: 12 mm, 1.6/2.4 grams
Bazaruco: 15 mm, 3.5/4.6 grams
2 Bazarucos: 24 mm, 6.2 grams
During the reign of Peter II, the diameters and weights were the ones I already gave for the first coin.
2. How do we read the dates on these coins---from left to right in each row (i.e., 1686 and 1619, respectively) or clockwise (1668 and 1691, respectively)?
Both systems were used. And to make things more interesting, sometimes some digits of the date are inverted.

3. I think I see "P" and "O", respectively, on either side of the coat of arms on the obverse of Coin 2. Do these stand for "Portugal"?
As Oesho said, the coins are Diu issues, and the P is realy a D.

Oesho, no Atias or 1/2 Atias were issued during the 17th century.
Lusomosa, thanks for the heads up, but you do have the same catalogs I have. Feeling lazy, are we?

Hope all of this helps.
Aldo