Each player allocates the hits and make saving throws in the same manner as they would when allocating hits from shooting.

Now I've always played this way: allocate hits to any units in CC first, then to units within 15 cm. If there are several units in CC (i.e. several "closest units"), their controller chooses which ones are hit.

I've played it both ways without issues.

I have always understood that "front-to-back" was relative to the enemy units causing the casualties, both for shooting and Assault.

This is why I tend to allocate hits from support fire relative to the supporting formation (within the 'assault zone' of course), which can mean allocating hits to a slightly different spectrum of units than in the original assault.

Hits are allocated ‘from the front to the back’ of a formation. Note
that this is the opposite of suppression. AP hits can only be
allocated against infantry units, and AT hits may only be
allocated against armoured vehicles. Hits must be allocated to
the closest potential target first.

3.3.2 Close Combat and Firefight Attacks
Instead of rolling a single hit dice for each war engine in an assault, roll a number of hit dice equal to the war engine’s starting damage capacity. You may choose to split these between close combat rolls and firefight rolls as you see fit, but close combat rolls will only hit enemy units in base contact, while firefight rolls will only hit units within 15cm that are not in base contact.

a) presumably, the third WE is blocked from taking art, because its LOS is blocked, and it cannot use the countercharge to move at right-angles (must move directly towards the enemy)

b) Can the two WE use FF to 'hose' the marines of the opposing tank? The stipulation in 3.3.2 seems to dictate otherwise

enemy units in base contact, while firefight rolls will only hit units within 15cm that are not in base contact.

On a related note, a slightly different question came up last night which I am sure has been answered before:-
Some assault marines make an assault on a an IG super-heavy tank company. The WE are in a straight line with the marines in B-B with two WE, and the third WE potentially isolated (M=marine, WE is the WE):-

M.W M.W W
M.E M.E E

a) presumably, the third WE is blocked from taking art, because its LOS is blocked, and it cannot use the countercharge to move at right-angles (must move directly towards the enemy)

b) Can the two WE use FF to 'hose' the marines of the opposing tank? The stipulation in 3.3.2 seems to dictate otherwise