turtle is behind the armorstand not infront of it:

Same happens when the slimeblock is in a players hand, so it has nothing to do with the armorstand

through a slime block item they are not rendered at all:


This helps testing:
/entitydata @e[type=Item] {PickupDelay:32767}

The second issue is MC-35920 (also affects chests and signs, but the topic does not state that yet). I doubt there is much Dan can do to fix that.

The first issue I have not see anywhere in vanilla, though similar issues exist making me doubt it is fixable without rewriting the rendering system,

Just tested this, was able to reproduce both problems with Turtles, Chests and Signs.
KnightMiner is right: this is due to the order that Vanilla sorts entities for rendering. 1.8 takes most rendering code out of the modders hands, so there's nothing I can do about this.