Given this combination of things, you should start by taking Corgan to your vet -- not because this is necessarily SM but because he seems to be bothered around the face and hindquarters. Be sure to print out the recommended documents on
www.smcavalier.com -- you will see which are suggested. Have a read through the "Is this SM?" section as it will list the things you need to try eliminating and address some of the questions you have asked.
If your vet cannot find any reason for these behaviours -- allergies, infections, blocked anal glands for example -- then you will probably want a referral to a neurologist familiar with SM. The neurologist will want to check for PSOM (glue ear in cavaliers) and SM. The only definitive way to diagnose is to MRI. There are some clinical tests the neurologist can try but a vet will not be able to make this judgement -- you need a specialist. However many dogs with SM will not be exhibiting the limb weakness and pain areas (sometimes, yet) that a neuro would check for in a clinical exam.
If you reach the point where you feel you need to see a neurologist, let me know and I will see if I can recommned someone in your region.
The most common misdiagnosis for SM is allergies. On average neurologists say it takes over a year for symptomatic dogs to get correctly diagnosed because vets will keep treating the dog for allergies. So if you are not getting any results from treating for allergies, it is a good idea to see a neurologist. One problem is that some of the things people give their dogs for allergies -- like Benadryl or low level steroids -- will also lessen the symptoms if it is SM so it can seem as if the 'allergies' are being addressed. However with SM usually the symptoms usually will eventually return and worsen so this is one indication that a neurologist should be contacted.
The SM infosite,
www.smcavalier.com, will answer a lot of your questions. I have pretty much most of what is known about this condition there including a long list of possible symptoms, Dr Clare Rusbridge's information sheets, information on PSOM, etc.
BTW it isn't abnormal for cavaliers to mush their face in the floor -- a lot do this. It's if it gets really odd or obsessive or seems to be because the dog is really bothered. Have a look at Chester's video in the videos section of the SM website and you can see a lot of the different typical behaviours.