Nose Object Problems
If breathing is difficult concentrate on the sensation at the focal point. Apply (Vitarka) Sustained (Vicara) Attention Directing your Mind (Manasikara) to keep your mind on the focal point to cut out Thinking and Pondering / Verbal Fabrication (Vachi Sankhāra). Even with a stuffed nose you still breath hence you can just concentrate in the in and out flow. I don't think it is very advisable to breath through the mouth through.

See my answer: https://buddhism.stackexchange.com/a/12342/295 which contains more  details.