Q: Can a 0cm move unit on a road claim the +5cm move bonus?
A: Yes and no. There is no clear distinction in the rules between 0cm move and truly immobile units. We recommend that units that could potentially be moved in an emergency situation, like field artillery pieces, be considered to have a move of 0cm. Truly immobile units such as a bunker in a scenario that has a unit data sheet should be considered as immobile under any circumstances.

A 0cm move unit would possibly benefit from the road movement bonus, allowing it to move 5cm per move. However, it may obviously not leave the road as it would then lose its bonus movement. Also, unit with a speed designated as “immobile” may not move, even on a road.

As always, if you anticipate any issue with this rule, for example you are playing Baran Siegemasters with many 0cm move units, go over it with your opponent during the 5 minute warm-up.

This would also solve the question "can landed aircraft garrison?", 0cm WE can't garrison as they can move, "immobile" WE can garrison.

I think it should be allowed (even though it wasn't the intent) and that it should be the distinction between 0cm move and "Immobile". You can drag/taxi/lift etc an artilery piece or aircraft but not a building. This would also solve the question "can landed aircraft garrison?", 0cm WE can't garrison as they can move, "immobile" WE can garrison.

4.2 AIRCRAFT
Aircraft formations are not set-up with other units. They
are kept off table (where it can be imagined they are in
orbit or stationed at a nearby airbase) but can be set up
on the board when they take an action. Aircraft that are
capable of transporting other units may be set up with the
these units already on board. The aircraft (plus any units
they are transporting) may only enter play using the
following rules.

• Any formations that include units with a speed of 0 (zero).

6.1.5 Garrisons says
so without a speed you can't garrison.

Wouldn't it be more intuitive to simply lose the distinction between 0cm and immobile?