Pebbles are uranium coated with multiple protective layers, extremely strong. They could be collected and reused if the launch vehicle plainly explodes on the launch pad.
As a result, the fusion "in 20 years" is much more plausible and feasible for the likes of SpaceX than fission.
No, because the effective specific impulse is so low, as a fusion reactor will only be able to fuse a small fraction of its mass before it is too radiation damaged to work. ITER, for example, would take 300,000 years to fuse its own mass in fusion fuel, but no DT reactor could operate more than a few years (if that) before parts need replacing.
If you want high effective specific impulse and high thrust at high specific impulse, beamed power is the way to go.
We should be able to beat fusion's Isp with direct matter to energy conversion.
Certainly the mass of fuel needed will be 3 OM lower.
Edit to add: Isp is a measure of velocity. The thing about velocity is, energy goes quadratically but momentum imparted to the vessel (the impulse) only goes linearly with velocity.
Since no process can be 100% efficient, you're dealing with quadratically more and more waste heat from the engines. And space is an excellent insulator.
There are people researching that too, but not as many as fusion. And well, determining the capacity of things that don't exist is the first step on the work of making them exist.