Panzym Concentrated Pancreatic Enzyme Powder

Yes, this is an over the counter supplement and it is safe for use without vet supervision. It should not cause harm to a healthy dog that it is fed it unnecessarily. However we strongly advise running it's use past your vet. EPI does usually require enzyme supplementation like this but starting without their consent may interfere with any diagnostic or treatment techniques that your vet has started.

Dosage:
Dog or Cat up to 10kg in bodyweight: Give a third to a half a teaspoon twice daily
Over 10kg in bodyweight: Give half to one teaspoon twice daily

This product contains enzymes that are naturally produced in the pancreas. Supplementation is advised for pets that lack the normal production of these. This is a supplement and not a medication so there are no listed side effects and it is generally very safe. Any possible adverse effects would be similar to that of a change in diet, for example if your pet is allergic or intolerant to one of the ingredients.

This supplement provides pancreatic enzymes in patients that are deficient in them. There is no strong evidence to say these enzymes will hep with acid reflux, the theory is that the reflux is caused by of a lack of enzymes naturally breaking down food. I recommend discussing this with your vet further, there are other ways to manage the condition that may be more beneficial.
If you are looking for digestive enzymes to supplement with the above in mind though then these are suitable.

A teaspoon of Panzym weighs apprroximately 2.6g so at a dose rate of half a teaspoon twice a day the 170g tub will last around 65 days.

Yes it is perfectly fine to give probiotics with this this supplement and is a good idea with this condition.

This product works directly on the individual meal it is given with, enabling the dog to digest this particular meal more effectively. It requires to be given in an adequate dose to do so. Therefore you should see some improvement in the faeces produced within 24 hours. There can be other complicating factors which prevent the digestion from being perfectly efficient as quickly as this, but your vet could advise you if any other measures are needed. It will obviously take a while for the average patient to put weight on, depending on lots of other factors. Usually some difference is noticed within 7-10 days.

Regretably this product is not likely to help your dog I feel. It
would not do any harm, but unless your dog has a digestive enzyme
deficiency (which is not typical with pancreatitis) then it is unlikely
to help. I would expect that any of the renal foods should be suitable
and it is very important that she is eating enough for her daily
requirements, so if she will not eat the ideal food, it is better to
allow some less than perfect food than she loses more weight through not
eating. Hill, Royal Canin and Purina all make good foods suitable for
dogs with chronic kidney disease and failing kidney function. If this is
what your dog is suffering from (+/- pacnreatitis) then I would suggest
trying a variety of any/all the renal foods, wet and dry. Obviously
check this with your vet who will be more familiar with your individual
dog, but generally speaking the above applies.
There are some other supplements which can help dogs with chronic renal failure, such as Pronefra and Rubenal so I would suggest looking at those too.