I agree with your points about why Jane might have fallen in love with Frank. And here's some evidence to support your idea that Jane was attracted to Frank's exuberant spirits:

'I did not make the allowances,' said she, 'which I ought to have done, for his temper and spirits -- his delightful spirits, and that gaiety, that playfulness of disposition, which, under any other circumstances, would, I am sure, have been as constantly bewitching to me, as they were at first.' Ch. 48

As for Jane's reserve, that does not mean her feelings don't run deep. She seems open enough in her conversation with Mrs. Weston after her engagement is out in the open. Also, she seems to share a lot of trivial things with Miss Bates like her real opinion of Mr. Dixon's looks, etc. I presume that her reserve does not extend to her close relatives like the Bates and friends like the Campbells. She must have been more open in her manners when Frank met her at Weymouth. Besides, it seems obvious that her increase in reserve when at Highbury (which even Mr. Knightley noticed) stems from her carrying such a burdensome secret.