Not much you can do at this point, as tuning is very hard to come by. Really, wait for an official fix from FCA, get emissions complaint tuning, or sell it. :(
You can use them anytime. As far as the best, that will be at the opinion of everyone. I personally don't run additives, and luckily I've never had issues not running them.
Most people are getting that lag.. as far as the settlement information, it just takes time. I'd recommend keep calling and ask to speak to a supervisor to escalate your claim.. exhaust all resources.