#1 Can I remove the brackets and place them on their side and under/in a bush next to my patio?
The bracket is removable. I would suggest that you bring them inside during rain or other inclement weather though.
#2 Can this be hooked up to a 2.1 bluetooth marine 4 channel amplifier receiver
yes the Polk Audio Atrium 4 can be connected to a Bluetooth amplifier. Be sure to check specs on the amp before doing so. The speakers are 8ohm and should be connected to an amp capable of 8ohms.
#3 Can you adjust the volume on the speakers instead of on the tv?
The speakers require something (usually a receiver) to provide power, this device would then control the speaker's volume.
#4 I will be running 100 watts a side and am concerned about the manufacturer's recommendation of 20-80 per side. Will I overwhelm them?
no you will not be over loading the speakers. Under powering speakers is a bigger concern. Under powering a speaker introduces distortion. Your amp will only feed what is required to the speakers. Having power reserves when needed is a good thing.