PNG screenshots are often several times larger than they need to be, and some upload forms will only take JPG.
JPG files are far smaller than PNG for photographs, which is usually why people convert. For a screenshot of text, PNG can actually look better, so convert only if you need to.
JPG cannot store transparency. Anything see through in the original comes out white, which matters for logos and cut out images.
Converting to JPG re-encodes the picture, so keep the quality high if the image matters. Converting back to PNG later will not recover what was lost.
No. It becomes white. Keep it as PNG or use WebP if transparency matters.
Yes, add as many as you like.
No.