Interesting topic the only lady I have known of who did the pre packed poos was domina Nia (her doggy bags as she called them)
I suspect the mistress in question is not domina Nia as you mention forced and that is not really nias style
However she talked regularly on her blogs of having fed people multiple doggy bags on top of fresh loads

I have had the pleasure of serving Nia and it was a pretty full on experience and although I only experienced fresh loads they were big and I can't imagine the people who managed 5 doggy bags on top of a couple of fresh loads!

Given she did it for a number of years I think it's safe to assume it was safe enough

Equally I remember many years ago on max fisch where somebody suggested the bacteria on a poo would double every hour and on that basis that would suggest that there would be an increased health risk though I believe people also suggested if it was frozen this would massively slow that process down.

Having had shit leftover from the start of a session fed to me to the end I can confirm that the longer you leave it the harder it gets to eat.

As for other ideas to make it worse I would say that domina Nia certainly had a nack of delivering a swift reminder if my enthusiasm wained and I for one can find if struggle to concentrate on the task in hand (well mouth) that multiple unrelenting kicks to the balls do refocus the mind

As for other ideas I remember this being discussed at max fish and there was some devious suggestions - most of which I can vouch for but some that sound very off-putting (to me though I know to others they may not)

1. Being used as a toilet after orgasm (I have done this and can confirm it is difficult)
2. Enema shit -this was on a recent post on here and I have always found this humiliating and if combined with a funnel can be a pretty long and difficult task
3. Being left to wallow in the stink and possibly not being allowed to shower - this would happen with Nia - anything you couldn't swallow properly and quickly you would be left in before though she would ensure you ate it all in the end and I was always dismissed without a shower and sometimes with a shit-tash to remind me of my need to improve
4. Restraint / gaggs - this can make things much more inescapable and although gaggs can make chewing and swallowing extra tricky being made to mash up shit with your tongue before swallowing takes a long time and is very humiliating
5. Being made to eat your own shit - this does not appeal to me but I know mistresses who insist this is a very humiliating act to put somebody through
6. I believe ms Billie posted on max fisch about making a shit and piss smoothie in a blender - I would have thought this should have made it easier to consume but she says the smell is off the charts and actually it's incredibly difficult
7. The diet of the mistress can make a huge difference - seafood, curry etc can make things much more difficult the next day from a toilets perspective
8. Shit up the nose - this is one that's been used on me a few times but a small amount of shit inserted in the nose to block your breathing but also gives such an intense smell and something that can be left in place through the session (blowing the nose or letting it fall resulting in a harsh punishment)
9. Adding tobasco or something else etc to any shit that is not consumed in say 5 mins
10. I once was looking at a long session with mistress Ming and she suggested starving me and forcing me to take Ipecac so that I would be deliberately starved and sick and very weak so I would be extra grateful for her shits - actually if she ever comes to Europe I think I will take her up on this
I find an audience humiliating - though a mistress once decided to facetime some of her friends as I was gagging on a log and showed me struggling to them- And somehow that was perhaps more humiliating than somebody being there
There was no other communication they just literally saw me gagging on a load and trying to beg for mercy with a mouthful of shit
There are some great posts on this forum about multiple sessions
Out of interest who is the mistress proposing doggy bags and Roman - she sounds fun