Are Blood Gemstones easy to come by and/or do they have unlimited uses

You don't ever lose it from dying, some enemies steal it. A couple areas are harder if you have 15 or more. It's pretty easy to get if you just want to spend it though.

No. Insight does not influence stock at all, it only unlocks the shop when you reach 10 ( 1 in latest version) Insight and lose it goes under.

If you are looking to unlock stock you get all of it by:

• Progressing in game
• Finishing Chalice Dungeon
• Killing few specific hunters

Slightly off topic lore side:

Insight has tiny effect on game difficulty and is mostly there to keep the player immersed into game's lore. According to the game's lore insight is gained by learning about the world and coming into contact with the Great Ones (basically seeing what you shouldn't see) and allows to perceive more things. It's sort of interesting how some enemies appear and other enemies start using new attacks, that they might have been there, but none of it hurt you because you couldn't perceive them. Also, too much insight enables perception beyond what people can handle therefore driving them insane and there are references scattered throughout the game.

At any rate, if you are interested in lore I am sure someone has analyzed and written about it.