I sure wouldn't put her pen over the sewer line. That is one "what if" that you might not want to tackle.
Hahaha - you want to take a guess how I know it's 30" down?? Been down that nasty road!
I also would not use the parameter fence as part of her pen. Then if she gets out, she is not OUT.
The spot I'm looking at is inside the pasture - where the inner edge of the pasture meets the yard & chicken coop. She'd be a good ways away from the perimeter fences.
My Pig Palace is in a corner of the outside fence, Pasture #1 fence and the garden fence. Over my DH's objections, I insisted on a whole new fence no less than 5' from any of the other fences.
For your shelter, you could build a Hawg Hut
https://www.theeasygarden.com/threads/hawg-hut-or-goat-or-sheep.18321/
I actually printed out some of this as a guide. But a hoop house would be good too for this season. What would you cover a hoop house in if not plastic/tarp?
Or you could build a hoop coop. Just don't tarp the roof, they tear and the pigs will rip them. The hoop coop might be better, make it on skids and you can move it if needed. Then you could build a mini-fence in case you needed to lock them up. If you have a boar, you will need a separate pen, also maybe a weaning pen...….With these short stubby guys you can probably use hog panels and they won't jump over them.