I haven't been able to figure out any combos with the Charge Blade other than X-X-X, but it's a fairly decent maneuver on it's own.
Sword and Shield Mode
I never use the A attack, since it's a slow one, but I assume I'll be finding a use for it later.
The X attack can be chained for a very fast 3 hits, and has a short cool down so you can just keep hitting.
You can also dodge in the middle of a combo or right after, and of course you can always use the shield to block attacks.
When the charge bar turns yellow, you can use shield+A to convert the charge into a useable form; 3 vials for yellow, at least 5 for red (but I think there's a maximum 5 vials, might be more for later blades). Don't ever let it get to "Red+Charge Warning", because all your attacks will bounce and stagger you until you convert the charge.
You can switch to axe mode in two ways. One is the default R+X, but the other is block an attack and while you're sliding back you press X and it will counter while transforming.
Axe Mode
Your R button here is used to switch back to sword mode. I've found that the attack used while doing this can easily be chained into a combo again.
When transforming, it will do a great big overhead swipe, OHKO-ing weak flying bugs and hitting most flying enemies. This can be combo'd into a X chain, OR an A chain immediately. The A chain when comboing directly from a transformation only uses two vials of charge to get to the most powerful strike in the chain, but is the hardest to aim that strike.
When I say the most powerful strike, I mean the hit at the end of an A-A-A combo (Or <R+X>-A-A combo) that transforms the blade automatically to a sword and shield. This transformation CANNOT be used to chain directly into another combo like the OTHER transformation back to SaS mode (above). However, it's still the strongest attack that deals the biggest explosion if you still have charge left.
So keeping in mind if you combo your phial strike immediately after transforming it only uses two phials, your A-A-A combo can only be done if you've transformed and let the blade come to rest. It's more maneuverable, because you can actually move to where the monster ran off to when you transformed, and you're more likely to hit, but in order to get to that strongest strike you're gonna use three phials, or more if a monster interrupted your combo.
General observations
Whether you mount with the Axe or SaS mode, it makes no difference for damage output or ability to stay on. When leaping off a cliff with your weapon put away, it will default to SaS. Mounting with weapon drawn requires dodging off the cliff, since you can't run or charge like a lance or hammer. Air attacks with the Axe have more reach and damage, but cannot charge the blade or use phials.
Auto-guard will often activate on attacks that don't normally deal damage, and use up stamina when you otherwise wouldn't. More often than it activates on attacks that DO deal you damage. However, that's useful for changing to Axe mode immediately.
The damage listed is slightly misleading, I think. It does seem to do more damage in Axe mode with X attacks then SaS with X attacks.