Now, is there a way to save this "Instrument" from one score to my overall Instrument List within VDL 2.5? As of now, this instrument can only be found in the particular score that it was created within.
That is a Sibelius limitation, instrument definitions confined to the score it's in. Otherwise, the only other way to move it from one score to another is Export/Import a House Style. Depending on how many modifications to things that have been done in either of the applicable files, this method could introduce "bugs" into the "new" file. May not be worth it to get just one instrument definition "migrated" to already-existing scores.
Once you have your modified "Master" template file, use that as your starting point for new projects.
Also, would be ideal for VDL to recognize the various multiple bass drum options and multiple staff lines as necessary to avoid notes appearing like cluster chords. Possibly even the use of a grand staff when 8 or more basses are asked for. The current configuration limits where notes can be placed on a 5 or 6-lined stave.
Yes, I agree that would be ideal, but grand staffs are only for pitched instruments - again, Sibelius limitation. Perhaps you could use multiple instances of the same instrument to alleviate the clustering issue (think "parts"). Just an idea.
Are you using a modified version of the BassLine 10-Drums Manual