Yep, it's all been asked before
but that's going to happen until nobody new wants to play this game, so welcome aboard and ask away...
Last question first...normally people play "what you see is what you get" (WYSIWYG), which avoids confusion. So those models with throwing spears would have them, and those that don't, don't. However, in a friendly game there's nothing wrong with saying all your infantry have them, or "all the guys with blue shields" or whatever. The point is you should make any non-WYSIWYG models obvious to your opponent.
In the rules for throwing weapons you'll see you can't use throwing spears as normal support spears, and a normal spear is not an option in a Rohan warrior's equipment list. Besides, it would just add to the confusion given the WYSIWYG issue above.
As for bow limit: it's 33% of warriors, heroes are not included. Riders and Rangers are the only models that have an issue with this, as they all come with bows as part of their standard equipment. With Riders the custom is to only allow those models posed as shooting to be able to use their bows, and to have the option of keeping them if they are dismounted.
With Rangers of Gondor or Arnor you can use the same method, but if you're playing Grey Company you have another option: if you take one Ranger of the North or Dunedain (minor heroes) per 4 Rangers of Arnor, then all your models can use their bows. It's effectively a 100% bow limit, but you take a bit of a numbers hit because you have to buy a minor hero to lead each squad of 4.
Hope that helps.
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