Why chose Spear instead of Spear and Shield? I understand that it may not be available in all cultures, but it is in the base rulebook.
But the base rules are simply the starting point. Other books will introduce other styles; there may well be a prehistoric settings that's pre-shield. Spear and Shield are in the rules because its an expected, common style for standard fantasy and, certainly, Glorantha.
What happens if a character has 'Sword and Dagger' at a different skill level than 'Sword and shield' and is only using a sword?
Its up to you whether you penalise or not, but I;d suggest he'd use his sword only at a level midway between the two skill levels.
If a character has 'sword' style and 'dagger' style can he use a sword and dagger together? If so at what level?
Sure, but if you feel there should be an off-hand penalty (because he's used to using his dagger in his main hand, rather than off-hand) - see previous answer for the skill level. Although, reasonably, why would have two separate styles? Why not just 'blades'?
Clockwork and Chivalry includes a combat style 'Black powder weapons', that includes reversing a musket and using it as a club. And yet a character with a huge skill in this will struggle with any other form of club.
Why? Let some common sense prevail. If you're trained in bludgeoning your enemies with a heavy club like object, why would you struggle with a purpose-carved club?
In the S&P article there is an example of a smaurai style which includes sword, great sword, bow, spear and musket. How many varying weapons can be included in one style? Is there any limit? If that many weapons can be incorporated in one style, why would a character have two or more styles? And is there no desire to differentiate between samurai characters with different levels of skill in different weapons?
Think about the martial training a culture will give its warriors.
All samurai
will be trained in these weapons, so there won't be that much differentiation. Ashigaru, though, will be trained in different weapons; as will Yamabushi. Think culturally.
As far as limitations are concerned, its whatever's reasonable for the style and profession. A medieval knight could expect to be taught sword, shield, mace and lance. A Roman gladiator might be trained in sword, trident, shield, net, spear and dagger. The aim isn't to balance out the number of weapons in a style to offer that flexibility without penalising characters in terms of skill picks and points.