What u have experienced is not common but not completely unusual either......shocks do fail, be they gas or oil ......perhaps its just bad luck u know....why dun u try and do a warranty claim from the people who sold it to u as 6 months is an unusually short time period for them to fail.
If they give any BS about u shouldnt have used them with eibachs or sumthing like that dont accept that coz the HD's are designed to work with any spring (within reason of course) otherwise Bilstein would have sold them complete with springs as a set...which they do with thier higher end range.
But as koryong said dunno if they will honour it....I think Mr. 535 had a similar problem and his warranty claim is still pending for very long time (think years not months).....
Conclsuion....u r kinda stuck....I'd suggest you dump the Bilstein's and replace em with ....another bilstein or something else, like u said TITANs perhaps.....dun ditch the eibachs tho....they are fine...u can use them with the TITANs unless u going for complete titan set or coilovers or sumthing...
If u wundering...Konis have a bad rep here in Bolehland....the legend or truth (depending on who u speak to) is that they like to leak....but in the US they are kinda popular....dinan offers them (with bilstein's), TC Kline (former racer turned tuner) offers them, and many other well know names....kinda odd dun u think? Maybe shiocks are made or valved differently according to the market....who knows?
Good luck!