How do I write a [B] flag for Rewriterules?

How do I write a B flag for Rewriterules for the URL below given that I can only rely on part of a URL param key substring "checkKey-" of checkKey-AlcoholDrugServices? (multiple categories can be chosen "checkKey-" is the common element)

I have a URL with GET vars I want to extract:

http://tsthealthpages.co.nz/index.php?option=com_mtree&cat_id=274&task=search&Itemid=null&checkKey-AlcoholDrugServices=Alcohol%20%26%20Drug%20Services&checkLoc-Canterbury=Canterbury

This is the output I get from print_r($_GET)

Array
(
    [option] => com_mtree
    [cat_id] => 274
    [task] => search
    [Itemid] => null
    [checkKey-AlcoholDrugServices] => Alcohol
    [checkLoc-Canterbury] => Canterbury
    [Drug_Services] =>
)

I want this output:

Array
(
    [option] => com_mtree
    [cat_id] => 274
    [task] => search
    [Itemid] => null
    [checkKey-AlcoholDrugServices] => Alcohol & Drug Services
    [checkLoc-Canterbury] => Canterbury
)

I think it has something to do with .htaccess not having a B flag for Rewriterules.
Example

As the problem I get is similar to :
"Given a search term of 'x & y/z', a browser will encode it as 'x%20%26%20y%2Fz', making the request 'search/x%20%26%20y%2Fz'. Without the B flag, this rewrite rule will map to 'search.php?term=x & y/z', which isn't a valid URL, and so would be encoded as search.php?term=x%20&y%2Fz=, which is not what was intended."

(The issue though is my example URL is not rewritten but the problem I face is similar, could possibly be a urldecode decoding the $_GET automatically but I have searched the code and been unable to find it)

I have posted another question in relation to this one:
https://www.experts-exchange.com/questions/28427882/How-do-I-json-encode-GET-vars.html?anchorAnswerId=40046749#a40046749

##
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AuthType Basic
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Options +FollowSymLinks

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# RewriteBase /

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RewriteCond %{REQUEST_URI} (/|\.php|\.html|\.htm|\.feed|\.pdf|\.raw|/[^.]*)$  [NC]
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