1. There are no special rules or practices regarding use of PDd shields in aggressive shield maneuvers. These shields are just as safe as any other.
2. The only construction use I've ever heard for PD in swords is to PD over the tip of a routed box (when sandwiching, not up-and-over type boxes), to help prevent separation and resist blow-out. I don't know if this is actually effective. See Adakan's stable tip design here for reference:

3. The use of spray PD vs paint on is objective. When using paint on you'll want to thin the PD with naphtha to allow for thinner coats to promote faster drying and a smoother finish. When using spray PD, you'll need to do light coats, and you'll have to do more coats than with paint on. You'll also need to account for overspray. Some find the spray on more useful for fine detail work for filling crevasses without leaving globs of PD, and then use paint on for everything else.
Though you didn't ask about it, option three would be to get a paint gun and air compressor and use paint on PD in it. Many PD artists that do commissions use this method and find it time saving and more effective. Typically they thin the paint on PD in a 1:1 ratio with naphtha, and clean their equipment regularly to prevent the PD from gumming up the works.
For additional assistance, I highly recommend joining the
Plasti Dip Artists page on Facebook. It's a collection of foamsmiths from various games that use PD in their work. Some of the best PD artists in the foam fighting community are members there, and there are handy tutorials and FAQs available there for new PD users.
Hope that helps!