The penalty is afaik between 25% and 35% malus to speed
and another between 25% and 35% malus to damage.
no you cant, some of them have cant use on horseback atributes , if they do it is written right where the stats are
Technically, you can use any 2h that can be used on horseback with one hand (although the wpf used is 2h). But only these can be used with a shield :
Note that you can even make a 1h swing with a 2h weapon (that can be used on horseback), on foot. Just chamber the swing and dismount. Also, no 2h weapon that can't be used with a shield can do overhead swings on horseback. Only 1/2h weps (usable with shield) and 1h can do overhead attacks on horseback.