1) This is going to be pretty setting dependent.
2) Mostly a balance issue. Otherwise, crafting is great for Clerics and Wizards, but nigh useless for Sorcerers.
3) Developer intent is that crafting should be an easy, routine task (which is why you can take 10 on the craft check). Pathfinder is basically set up on the assumption that you're playing in a fairly high magic setting. That assumption permeates the game all the way to how CRs are calculated. It'll take a some care and house rules to adjust everything for low magic. Not a huge deal, I don't think, but it's important to keep in mind.
4) The 50% discount basically works out to give the crafter an all around +1 to stats covered by each crafting feat. So, Craft Magical Arms and Armor gives about +1 damage, attack rolls, and AC compared to a non-crafter who spent the equivalent amount of resources on weapons and armor. This put the crafting feats pretty much in line with other feats, but with the additional requirements of time to craft gear and continual investment as the character levels to keep the bonuses. Compare to Power Attack, which scales automatically as you level and gives you the full benefit immediately.
I think the time is the major factor that may be overlooked... At best, that Robe of the Archmagi is going to take 8 hours a day for 33 days to craft. Unless your crafter is making good use of a Ring of Sustenance and has a safe place (such as Mage's Magnificent Mansion) to craft at night, that's going to mean a lot of downtime in towns. Of course, the world around the players shouldn't stop moving just because they take downtime... The villain is likely getting stronger, or burning down more villages, or whatever. Oftentimes, crafters just plain won't have enough time to get the full theoretical benefit out of their crafting feats.
4) What is the rationale for making magic items so easy to create? One feat and BAM a wizard has a 50% discount and is his/her own magic item shop (I don't think one or two feats will impact a wizard very much anyway).
Feats are just as important to a caster as they are to any character type, and crafting magic items requires a different feat for each category of item. Granted, Craft Wondrous Item covers most of the body slots, but important items like metamagic rods, staves at higher levels, etc. are left out.
50% is an interesting number. In addition to being the craft-cost of magic items, it's the sell-price of magic items found while adventuring. Since most of the value in treasure comes from magic items, and they constitute the vast majority of what PCs want to acquire, magic items constitute the base of the party economy.
- Any item the party finds and uses has not been crafted by them.
- Any item the party finds and sells is worth half price.
- Any item the party crafts themselves costs half price.
On a financial level, crafting feats simply convert a portion of unusable treasure into usable treasure of the same value.
Good info