When will Pexion fully work for my dog's shuffling behaviors?

You are correct that both of these medications are used to treat epilepsy in dogs. From the label of Pexion: "The pharmacological response to imepitoin (Pexion) may vary and efficacy may not be complete. Imepitoin is contraindicated in patients hypersensitive to it, with severely impaired hepatic function or severe renal or cardiovascular disorders." However, I could not find information that hyperactivity was a side effect of this medication. It could be that Jasmine is feeling uninhibited on this medication and thus does not know how to control her hyperactivity. A common side effect of Epiphen (phenobarbital) is sedation, but this effect commonly wears of with continued used. I could not find information on the same effect when using Pexion. Not knowing your dog's complete medical history and particular reasons for choosing anti-epileptic medications, I cannot make specific recommendations for her. I would recommend discussing these effects with your veterinarian. You may consider a consultation with a board certified veterinary behaviorist, or medications used for anxiety instead of seizures. Common supplements used for anxiety include Zylkene and Comfort Zone Adaptil Collars or Diffuser.

The dosage of 100 mg twice daily is well within the dosage guidelines. The recommended dosage is 10-30 mg/lb twice daily. You dog is approximately 15 lbs, so the maximum dose for that weight is 450 mg twice daily.

A single dose appropriate for her weight is unlikely to cause any severe effects as long as she has no additional health concerns. Monitor closely for any altered behaviour or other symptoms but these would be unlikely

I think many veterinarians would continue with both medications if the seizures were refractory to the first medication, but other times the goal is to try to transition from one anticonvulsant to another for various reasons. I would recommend asking the specialist or your veterinarian (whoever will be managing Jig’s seizures) what the long term plan is.

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The active ingredient in pexion can have a few mild and usually temporary side effects such as increased thirst, hyperactivity, hypersalivation etc. I have never heard of it causing breathing issues explicitly, but it could be a side effect. I wouldnt worry about it overly though as it sounds like Jasmine is ok orherwise.

I usually don't expect to see positive changes for at least 6 weeks. You may want to discuss these side effects with your vet to see if there might be a better anxiety medication to use - there is no one best medication for all dogs, sometimes it is trial and error. I can't really tell you what to expect next, since every dog is so different. I would not expect that his current side effects would get worse, though.