The best time to transplant is generally either fall or spring.
Here are two articles to help:
https://www.gardeningknowhow.com/ornamental/shrubs/shgen/best-times-for-transplanting.htm
https://www.gardeningknowhow.com/ornamental/shrubs/potentilla/growing-potentilla-shrubs.htm
No image needed
It can tolerate acidic to alkaline soils with a pH between 5.5 to 8.0.
https://www.gardeningknowhow.com/ornamental/shrubs/potentilla/growing-potentilla-shrubs.htm
Are Potertille plans dear resistant?
There is nothing that is, truly, deer resistant, unfortunately. There are plants that they do not prefer, but they will nibble on, just about, anything.
This article will help you with deer proofing:
https://www.gardeningknowhow.com/plant-problems/pests/deer/deer-resistant-garden-plans.htm
Bushes overtaken by grass. How to get rid of grass?
There are a few ways to do so. You can choose chemicals (Glyphosate will be a fast, spot treatment.), or natural methods. Laying cardboard and wetting it, while allowing it to remain for a few weeks will kill the grass. Just be sure to apply a fungicide before you do so, or it could invite infections to the area.
Here are some articles that will help:
https://www.gardeningknowhow.com/plant-problems/weeds/glyphosate-information.htm
https://www.gardeningknowhow.com/plant-problems/weeds/grass-growing-in-flower-bed.htm
Want to plant next to ocean in Maine.
You are in hardiness zones 5 to 6. Potentilla is hardy to zone 2 so it should be fine planted in your landscape.
Should the dried-up blooms on the Goldfinger Potentilla be deadheaded off to encourage more blooming or just left on to fall off by themselves?
You can leave them to fall off by themselves as Goldfinger is considered self cleaning. It should bloom all summer. But if you like a tidier appearance, go ahead and trim the dead blooms.