There are not really any safe doses of Benadryl for infants that young. Cough syrup is not recommeded without the advice of your physician and they recommend the same with antihistimines. You best bet is some Tylenol infant drops and a humidifer, or run a steamy shower and take her in there to relieve the congestion and if this does not relieve the symptoms it is better she be seen for an examination at her age. I have included a link below that explaines about all the different types of medications you have asked about that I feel will be of help to you. If you have any further questions please feel free to ask.
http://www.medem.com/medlb/article_detaillb.cfm?article_ID=ZZZJXPZTR7C&sub_cat=409
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