Hi, welcome to BW
First off, she's very young. She could be crying for a multitude of reasons - one of which may be that she just wants to go to sleep and will you please put her down or she may not be tired enough for a nap yet.
Can you post your EAS? That will help - if she's over or undertired, she may be unhappy about shush/pat and knowing she's going to bed and you want her to sleep.
1. What to do if DD starts fussing / crying during sitting? Do I let her cry while staying still? Do I pat? Walk? If I just stay still. How long does it usually take for them to stop crying?
I would probably be patting and shushing whilst sitting then continue that as you put her down into her bed so there's something bridging the transition but I wonder if maybe your wind down is too long or too early.
Usually you can tell if LO is going to settle or not - if you don't think she'll settle, I'd just find another way for her to sleep.