i bought my 100g purigen in a pouch, like regular media
i'm sure you could fit it in your filter as the bag is quite small. You can still buy a bottle of granules and put them in a filter bag. Seachem sells a filter bag but i'm sure aquaclear filter bad or nylon stockings would do the job
I use AquaPlus to dechlorinate my water and have had no issue with it's effectiveness.
to regenerate it, you dip in a 1:1 water:bleach solution for 24 hours.
Then you rinse it and let it soak in water for 8 hours.
I think the pH buffer they are talking about is to make sure the bleach is fully removed as bleach will alter your pH. I've only used it for a month so i haven't gone through this yet but i'll post here when i do so.
As for the dechlorinator bit, this is probably seachem trying to sell their own dechlorinator so they won't guarantee the results with another brand...