Your concern for your two friends is quite admirable. I must say that M seems to have been MOST cruelly treated by W. It sounds like M has a true friend in you, and that she has found a real gem in the colonel.

I think you have answered your own questions:

FIRST:
Should M more honestly appraise her feelings for him (the colonel)?

She has admitted to being touched by his qualities and his love for her and that the colonel stood by her and did the best he could for her so that M has intimate knowledge of his goodness. Love doesn't get much better than that, I think you'll agree.

SECOND:
If they marry, will their marriage be a stable one?

You said I have no doubt that, because she cannot love by halves, her heart is quite capable of becoming in time as much devoted to him as it was to W. M is already in love with the colonel; she just hasn't said the words yet! Give her time; the words will come!

You have nothing to worry about. They have all the makings of a good marriage, and time will prove that, to you, and to her.

All the best to you and to them,
Agony Aunt