SlikSLIK DQ-10 Mini Quick Release Adapter Set - Small, Black (618-743)

Q: Is the plate arca compatible?

No. Silk uses their own design.

Q: Is the bottom of the plate flush?? Looking for a good option to use with my Zhiyuan Crane

This thing is flush on BOTH sides. The cork part your camera sits on AND the part that sits on you tripod or other mount. Even the release handle fits between the two surfaces and does not interfere. I have one of these on a permanent windshield mount for an action cam I am using as a dash cam. Makes it SO easy to remove the camera for security reasons. I would have posted pictures but there doesn't seem to be a way to post them on this answer.

Q: What is the width and length of the base of the mount (not the plate, but the base), please? (amazon often gives dimensions of the item's box.)

The part that screws on to the tripod is 1 3/8 inches by 2 3/8 inches (the long part was measured in the middle where the latch is located.) The part that screws on the camera is 1 1/8 inch by 1 1/2 inch.

Q: would it be solid enough to hold a camera across the short part of the plate, not along the long side? camera is about 300 grams (10 oz)

A: I would think it would be strong enough. Although I can't really imagine why you would have to do that. This mount will never be stronger and more stable than the way it was designed to be mounted. Also..another reason for the design is to ensure that the base plate that goes under your camera and the receiver for flush under the camera body so there would be little or no obstruction in camera function or the tripods abilities. Most tripods can tilt your camera to virtually every angle so depending on the camera body, if you do not use the quick release the way it was designed you may or may not be contributing to more stress on the base of the camera..