Walking up the aisle
I think your moh is fine to walk alone. As she's your moh rather than a 'bridesmaid' then I don't think it will look weird at all.
I think with when you start walking down the aisle, you can play it by ear. You don't know how long it will take them all to walk that distance so maybe just see if there's a particular point in the song on the day that sounds right as long as it's not too far behind the rest of the bridal party.