"Can you bring a civil case to reclaim them then if the action was unreasonable.."
Theoretically, yes, but there is only one such case seeking damages after an RDNH ruling of which I'm aware, and it was only recently filed. Ask again in a few weeks.
"What sort of costs are likely to be incurred?"
Mailing the hard copy of the response to the complaint and the dispute resolution provider... and/or faxing it. Maybe obtaining a copy of the complainant's tm file from the USPTO... photocopies... There are no fees assessed UNLESS you want a three member panel and the complainant has not already paid for a three member panel. In that case, you will pay around $1500, depending on the provider.
" ie: can you defend yourself"
Yes. You can fix your own plumbing too, if you are willing to live with the results of your plumbing skills and experience.
I recently did minor surgery on my hand, and it came out okay!