can I use cables that 'convert' balanced to unbalanced?
Yes, depending on your environment. As Mark said, the entire length of the cable is unbalanced, so you lose the noise rejection that a balanced link would give you. If you're not running power cables etc. near your audio lines you can get away with an unbalanced connection. I wouldn't rely on this in a setting where you regularly have to move your system (PA hire for example).
Can I assume that such cables are bidirectional in that if they are listed as unbalanced to balanced, they will also do it the other way around?
No. Those cables just make your link unbalanced in both directions.
The Canford converters you linked to, are bidirectional. They use a different method (a transformer) that actually converts the signal. This is basically a passive DI. If you place this converter at the end of a balanced link, you preserve the balanced link and its noise rejection.
As an alternative, an active DI would also work, but those have to be powered. They're usually built to take phantom power or a battery, I haven't seen DIs that can be mains-powered.
Note that in the same form factor as the Canford converter, you can also get converters that just connect pin 1 to 3 (as in the "pseudo-balanced" cable you linked to).