[Lord Melkor]

Hmmm, are you sure these three roads are correct, Raynor? If I read the quote you posted correctly it discusses several things.

1. Why refusal of the Summons of Mandos occurred less frequently in ancient days:

It was less frequent [to refuse the summons of Mandos], however, in ancient days, while Morgoth was in Arda, or his servant Sauron after him; for then the fea unbodied would flee in terror of the Shadow to any refuge

If I read this quote correctly I take it to mean that the unbodied fea would flee the terror of the Shadow to the safety of the Halls of Mandos.

2. Exceptions to the previous rule:

...unless it were already committed to the Darkness and passed then into its dominion.

Unbodied fea who were committed to Darkness would refuse the summons of Mandos and would instead fall under the dominion of the Shadow.

3. What happened to corrupted Eldar.

It is important to note that the previous lines primarily discussed what happened to the fea of the Avari, the Elves that refused to go West, which becomes clear if we look at the preceding sentences.

Among those who refused the summons (or rather the invitation) of the Valar to Aman in the first years of the elves, refusal of the summons to Mandos and the Halls of Waiting is, the Eldar say, frequent. It was less frequent [to refuse the summons of Mandos], however, in ancient days, while Morgoth was in Arda,....

Therefore the line:

In like manner even of the Eldar some who had become corrupted refused the summons, and then had little power to resist the counter-summons of Morgoth.

is not really a 'third option' but is simply to illustrate that even one of the Eldar would not be able to resist the counter-summons of Morgoth if he refused the summons to Mandos, just like the Avari from the preceding sentence.