You raise a number of issues. If the tagline is currently registered, you should treat it as being in use until you have proof to the contrary. You should check to see when they filed their last renewal. This will give you some idea of when they were last required to provide evidence of use.
As for the more specific tagline, the "The Fund that", the question would be whether that tagline would cause confusion with the registered tagline.
As for purchasing the registration, it is always an option. You could give them a call and see what happens. Obviously, by calling they may feel that it is worth more than before you called, thus raising the ante.
This situation may need more attention than this board can provide. You may consider seeking a tm attorney to help you sort things out. (Where is Josh when you need a free consultation.....
I hope this helps.