Upload forms have limits and mail has limits, and the number people actually have in their head is “under 2 MB”, not “quality 70”. Say the size; the tool searches for the best quality that fits under it.
Result
images
Because the size is the number you have. A form says 2 MB, a mail server says 10 MB, and nobody knows which quality setting produces that for their particular photograph — it is different for every image. The tool encodes several times and keeps the best one that fits, which is the search you would otherwise be doing by hand.
Because compressing it again would have made it worse without making it meaningfully smaller. A photo that has already been through a compressor has already lost what there was to lose, and re-encoding mostly adds new artefacts. Most tools will happily hand you a slightly larger file and call it a saving; this one says so instead.
No. The decoding, every re-encode and the result all happen in this tab. That matters more here than it looks: the photographs people compress to fit an upload form are usually documents — a passport page, a payslip, a signed contract.
Yes, and each one is searched separately, because the quality that fits a size is a property of the individual picture. The result names any that could not be improved rather than quietly leaving them out.