ksc,
1 Yes, all of the versions of the QuadraBeam transducer are usable with the 998 units. Usually we do not list a unit as being compatible with a transducer if it does not allow all of the sonar functions of the original transducer to operate. Since the QuadraBeam transducer does not have Si sonar nor the 800kHz capability, it is not listed.
2 Both transducers have the same internal components and will work the same. The only difference is in how they mount to the boat, what they are made of, and what installation hardware and installation instructions are shipped out with them.
3 I believe that the limit to the 455kHz Side Beams for the QuadraBeam transducer is 150 feet as well. In the right conditions it may work past that depth but than so might the Si beams of the HDSi transducer no guarantees though.