1. To prevent any interference with the auto Electronics regardless if the radio is floating or not is to go to the point where the battery terminates to the vehicle. Typically they have 2 termination points. 1 to the alternator and another to the engine block, and another smaller one to the firewall. Use the one with the heaviest cable.
2. I completely disagree with fusing the negative lead. If for some reason the fuse were to open, would cause the return current to flow through something it is not suppose to like the coax shield or mic cable. In which case would likely burn open a solder tract inside the radio. The negative should be a solid made connection whether or not you take it to the battery or the chassis.
Personally when I did installs in the 80's after High Energy Ignitions and ECM came out, we used a special battery replacement bolt distributed by Motorola. It replaced the factory battery bolts for side mounted battery post. I did litterally hundreds of Law Enforcement vehicles and worked closely with the auto and radio manufactures. I was even published in a trade magazine called Mobile Radio Technology for reducing noise interference in mobile installations in about 1984. (I think it was 1984, been a long time). It was my MSEE thesis paper in college.
Most of today's vehicles now come with a factory battery term plate on both the battery post with an empty stud positions to facilitate high power accessory equipment terminations like two-way radios and high power stereos.