] I think Colonel Brandon felt genuine affection and devotion to her. Certainly,Elinor observes his emotion on seeing Marianne's altered looks as misery which must arise from more than his feelings for Marianne. Yet, the Colonel seems a sensitive guy who is more complex than to simply marry M. as he sees her as an incarnation of Eliza.

...as to marry a woman who mearly resembled his first love. That would be beastly.
As for loving Marianne for herself, the novel speaks of his "strengthening regard" for her, and he himself says that had Eliza been guided by a firmer mind, (such as Marianne posseses) she would have been as happy as he knows Marianne will someday be. He speaks of her fortitude, in contrast to the statement "though she promised me that nothing...(would overcome her resolution)" - speaking of when Eliza promised him she would wait for him, then married his brother.
All this is strong evidence that though he may have initially been attracted to Marianne because of her resemblence to Eliza, he realizes that Marianne posseses a stronger mind and a strength of heart that Eliza never had, and loves her the more for it.