Question on Mile'eil/Milera

I've been having a discussion with family members about pronunciation of certain pairs of words that have a Makaf, and the second word of the pair is either monosyllabic or is Mile'eil with 2 syllables. The rule is that the first word also becomes Mile'eil, even if it would have otherwise been Milera.

Examples:
ופני רשעים תשאו-סלה
עיני-כל אליך ישברו

That much is clear.

However, what happens when the second-to-last syllable on the first word turns out to be a שוא נע or a חטף-פתח, חטף-קמץ, etc.? I was taught long ago that an accent cannot be on a שוא or a חטף-פתח, חטף-קמץ, etc., but perhaps that doesn't apply here. Or maybe it does.

Examples:
מְשַׂמְּחֵי־לֵב (from תהילים יט)
כְמוֹ־נֵד (from אז ישיר)
כְּמוֹ־אָֽבֶן (from אז ישיר)
אֲשֶׁר־שַׂמְתִּי (from שמות טו)
כְּבֽוֹד־אֵל (from תהילים יט)
עֲלֵי־דֶשֶׁא (from שירת האזינו)
סְלַֽח־נָא (from במדבר יד)

Which of the following is the rule?

a) Keep it Mile'eil regardless of its vowel
b) Make it Milera in such cases
c) Make it Milera only when there are 2 syllables, but otherwise, put the accent on the third-to-last syllable (e.g. משמחי-לב)
d) Other rule ___________
e) No rule - you just need to know the Mesora for each instance

I'm leaning toward "C", but I'd like to hear other opinions.
(For סלח-נא, one Tikkun I saw puts the accent on the ס while one Tikkun puts it on the ל, so I'm just confused.)