1. Tattoo; My understanding is that for a crucifix to be crucifix, it must be blessed by a priest. Now, I suspect that detail has been lost or forgotten over the ages, but there are sacred tattoo artists out there. There is a family that has been tattooing pilgrims to the holy land since the crusades, as well as Samoans practices. But that is where I would go with this, a Slayer should use the holy symbols of her culture. If her culture used tattoos to ward off evil, then do it, but for slayers based in Christian Europe, crosses will suffice.
Lore wise, Lindsay was able to use mystical tattoos to hide from the Senior Partners, so such magic could exist as an anti-vampire ward.
2. My understanding is that the games stated that the X-bow bolts were dipped in holy water. Remember, Buffy has a big chest of holy objects that we never see her use, and we know she's gone to get water blessed before. I just chalk it up to the time she and Giles have to spend on cleaning and preparing her weapons and herself for combat every night. Which would have been a funny scene.
Giles: "Sunset is in three hours, I trust you'll want to clean up, go to confession and enjoy a good meal, that sort of thing.
Buffy: "Sure, pre game rituals."
***
Giles: "Ah Buffy, how was your 'pre-game' rituals?
Buffy: "I ate half a bag of corn chips and fooled around with Willow. She's trying to convert me."
Giles: "What?'
Buffy: "To Jewish...-ness. Her rabbi was nice, apparently one of those Jewish traditions that's very strict on Christmas specials but surprisingly open minded on everything else. But, I'm here to slay!