The noise level with the exhaust will be down to the manufacturer, you can have a decat exhaust with a large silencer box and keep the levels low, conversely you can have hi flow sport cats fitted followed by a straight through pipe and it will be louder than your standard setup.
from the diagrams it looks like the lamba is just after the first cat
I cant help with rear plug access on your vehicle, but Im sure one of the others will be along with the knowledge