I'm focusing strictly on what we know from the text up to the end of Ch10, and will take your points in order.

“..Captain Wentworth was not in love with either..”

This is Anne's point of view and not necessarily accurate. Anne may have understood FW very well in 1806 but she does not know what he is thinking now – she can't tell which sister he prefers or even whether the lively or the quieter character is more likely to attract him. When he was an impetuous young man he tried the “falling passionately in love” approach, and look how that worked out! Perhaps now he is open to a less interesting form of attachment – he has certainly given the matter a lot of thought and does not want a re-run of the events of 1806. Even if Anne is right that FW is not in love with either sister, it does not follow that he never will be. It might be more accurate to say that he is not yet in love..

“The Miss Musgroves' little fever of admiration might, probably must end in love with some.”

Their fever of admiration reminds me of the sort of crushes teenagers gave on film stars or the captain of the football squad. Henrietta and Louisa seem to be more frivolous and less serious than Anne, and their feelings will not automatically follow the same curve as hers did. I also think that the young women have some responsibility for guarding their behaviour and for being cautious about allowing their feelings to become too much engaged. Perhaps there is some merit in the idea that a young lady should not dream of a gentleman before the gentleman is first known to have dreamt of her.

“Everything now marked out Louisa......”

The passage in your third quote is the first instance I can find where Wentworth singles out Louisa, rather than allows himself to be singled out by her. I agree that appearances mattered, and it seems likely that FW is somewhat naïve about the ease with which people might form ideas of an attachment where none exists.

Suppose FW is not yet “in love” but is still open to the possibility of his relationship with Louisa developing into a serious attachment. What exactly should he have done on the walk home from Winthrop? Henrietta and Charles Hayter were acting very much as a couple at this point, so FW would have been de trop if he joined them. When they set out Charles is sandwiched between Mary and Anne who has been apparently giving him the cold shoulder for weeks. FW is more or less obliged to walk with Louisa or be seen to be snubbing her. When they reach the end of the meadow and meet the Admiral and Mrs Croft, FW does detach himself from Louisa and help Anne into the gig. I suppose he could have abandoned her earlier and made a point of joining Charles Musgrove in his hunt for a weasel, but if this lapse is enough to condemn FW's behaviour, then I begin to think that any eligible man would have done well to cultivate a Darcy-like reserve when in the presence of a marriageable female!