I've had three long haired cats in my life - two of them kept their fur in beautiful shape without me ever having to brush or comb them.
The third one is my "special" needs kitty, and has never had the ability or gotten the concept of grooming. She's been shaved once - by the vet, who put her under for the procedure (she was shaved on the belly, neck, and butt region). I use a slicker brush and mat comb on her now, occasionally cut out mats, and keep certain areas trimmed all the time.
I'd start fresh - get her belly shaved, and then I'd highly recommend using a slicker brush on her - it pulls out a TON of that super fluffy undercoat. Truly works wonders on Pfeffa!