Lightroom :: Deleting Multiple Keywords In LR5

I am importing about 20,000 pictures from Flickr. they are all creative commons, and the end result will be the use of approximately 4000 of these in an online sign language dictionary. I use Bulkr to import them and this process imports the tags as well as other information such as the photographers name and url.  Many of these photos arrive with hundreds of tags, so that the total number of tags might be 20000 or more.  I need to assign my own tags and delete all the tags that the photo arrived with.  I use the lightroom batch delete to do this (the little minus sign at the top of the keyword list).  i select a keyword, hold down the shift key and scroll down.  the problem is that i cannot just scroll down to the bottom of the list.  There appear to be unpredictable keywords that cause the shift-select process not to work.  if i scroll back up and then carefully scroll part of the way back down, then there will be a point at which shift-select stops working and above that it works.  The end result of this is that it takes numerous shift-select-scroll down-select to delete everything so that what should take maybe 2 minutes takes 20 minutes or more. 

The problem may be related to foreign language keywords especially chinese characters, but even what looks like a perfectly ordinary keyword can stop it.  Two recent examples: travelers cheque  and Київ. In all cases, the words can be deleted when i click directly on them.  you just cant shift-select THROUGH them.A separate annoyance that i will just grumble about is that i was forced to put these pictures in a separate catalog because there is always the risk that i could manage to delete my own keywords while batch-deleting everyone elses, and there is no simple way that i know of to undo this. with the separate catalog at least i wont delete all the keywords for my own pictures.