Learn French: Deux d'entre eux sont loyaux.

In French, you cannot put de directly in front of a stressed pronoun like eux, nous, or vous to say "of them" or "of us" after a number or quantity. You must use the construction d'entre (literally "of among").

Would it change if the "them" referred entirely to a group of women?

Yes. eux is the masculine or mixed-gender plural stressed pronoun. If the group were entirely female, you would use elles, making the phrase deux d'entre elles.

Why is the adjective spelled loyaux instead of loyals?

Most French adjectives ending in -al form their masculine plural by dropping the -al and adding -aux. Because eux indicates a masculine or mixed group, the masculine singular loyal changes to loyaux. If the subject were feminine, it would be loyales.

What is the exact subject of the verb sont in this sentence?

The main subject is the number Deux (Two). Even though d'entre eux modifies it, the number itself acts as a plural pronoun here, which is why the verb takes the plural form sont.