With respect to the fuse in the negative questions is straightforward. It protects nothing. Remove the fuse, and the radio still operates because it is using the coax shield for DC power. The fuse that protects both positive and negative wires is the positive fuse. It is a Grounded system where one circuit conductor is bonded to Ground like a frame of a vehicle. In an AC system, the grounded circuit conductor is Neutral.
The whole point of a Grounded System is to facilitate simple, cost-efficient, and effective over-current protection of the circuit conductors. Only requires the ungrounded circuit conductors to have a breaker or fuse located at the feed point.
A Floating System like batteries or an AC Delta circuit where no circuit conductors are bonded to ground requires a fuse or breaker on all ungrounded circuit conductors, plus ground fault detection in the event one of the circuit conductors is grounded. If you FLOAT a battery system would require both positive and negative conductors to have an over-current protection device installed.
Over Current Protection Devices (OCPD's) are not used to protect the load utilization equipment like your radios. Fuses and breakers are there to protect the wire connected to the load side terminal of the OCPD. Nothing else. Anything beyond that is collateral. All electrical codes have minimum size wire requirements based on the breaker Ampere rating. For example, a 20-amp OCPD requires a minimum of 12 AWG or larger wire. The insulation on a 12 AWG wire fed with a 20-amp breaker will never experience thermal damage to the wire's insulation preventing an electrical short or fire as a result. Electrical codes protect the power wiring only. What you plug in is an Equipment Code issue equipment manufacturers deal with.
Electrical codes do not permit grounded circuit conductors to be switched, or have fuses and switches inserted. They must be solidly bonded and never interrupted. The reason is very simple. For sake of argument imagine this. Disconnect the radio coax from your radio, and assume your installation did not use any chassis-to-chassis bonding and is electrically isolated and floating. You slid the radio under the seat with carpet and rubber backing. Now pull the negative fuse while leaving the positive connected normally. Take a DMM and you will read 12 volts on the radio chassis and negative wire where you pulled the fuse. It is looking for a return path to the battery. The radio chassis is HOT.
Granted 12-volts is not a shocking experience. Now repeat the experiment with a higher voltage, say 120 volts. Real simple to do using an old fashion incandescent hand test lamp like you would hang under the hood of a car while working; a test plug for an AC wall socket; and one test jumper with alligator clips. Use the test plug and jumper to connect Line to the lamp power plug line stab. Leave Neutral circuit open(DC negative equivalent). Turn the lamp switch on. The light will not come on because Neutral is open. Now wet both index fingers on each hand. Take one wet finger and place it on ground, and the other to the lamp power plug neutral stab and watch the light come on and lights you up.
Now imagine Mr Sparky, your friendly Electrician working in Sammy Hammy's house with a low voltage problem in the shack. Mr Sparky checks your AC power at the wall receptacle. Like all Mr. Sparky's know, Neutral is grounded and safe to touch. While checking your AC wall circuit, he jumps and cusses because he got bit with 120-VAC touching the Neutral terminal. Checks the wiring and finds Sammy did some wiring work and put the AC wall receptacle on a wall switch switching the Neutral rather than Line. Mr. Sparky finds two problems. Undersized wire causing the voltage sag problem and improperly wired wall switch. He knows why electrical codes do not allow the grounded circuit conductor to be interrupted and must be solidly bonded.
Connect your radio's negative power wire to the vehicle chassis as close to the radio as possible using a hard point like a seat bolt or ground terminals provided by the auto manufacture. They are under seats for things like Inverters, high power audio amps, heated/power seats, and accessories. Do not roll up the slack on positive, negative, and coax. Keep them as short as possible. Otherwise adds voltage loss and noise. Throw the second fuse in the parts bin.
Manufactures provide the second fuse in the event sammy hammy installs the radio with negative power wire terminated directly the battery term post. It protects sammy hammy from himself in the event sammy hammy worked on the car and forgot to reconnect the vehicles factory bonding jumper back up. Pros call them dummy fuses or spare parts.