Okay, so I got a little bit of relevant data from Dudek. On the chance it'll help someone else in similar situation, they say: "For Powerseat Comfort S / M, the container capacity is: min 4300 cm3, max > 5400 cm3, and for L / XL it is min 4600 cm3, max 5800 cm3." That's not a perfect guarantee this or that reserve will fit, but it gets us much closer.
Yes once the velcro is opened on the ground you can re-do/tuck it away so the tip steer toggles are just held on the magnets. They are plenty strong enough.
Your thinking on the speedbar is correct, depends how you want to fly the glider. If you have trimmers and speedbar active then you should only use the tip steer