I have told you on multiple occasions what would convince me that there is a case for design: evidence, i.e.:
1. A visit or communication from an extra-terrestrial intelligence describing how it seeded the Earth to create life.
2. Identification of a biological organism that could not possibly be constructed via small incremental changes.
3. Identification of a present-day biological organism that shares no common traits with any other known organism, and has absolutely no prior fossil ancestry.
4. Discovery of some feature in the replicating material of a biological organism which communicates something other than the information necessary to cause the organism to replicate (e.g., a copyright, trademark, signature, logo, or other marking).
There are other possible evidences which I would personally view as sufficient to cause a scientific search for design. But, they all share one common theme: a physical manifestation which can be scientifically measured.
Absent such a measurable trait, no measurable evidence exists, therefore ID is no more scientifically tenable than is astrology -- in fact ID is less tenable, because at least astrology claims something which can be scientifically measured (i.e., predictability). Whereas ID claims nothing which can be measured -- it makes the bare claim of its truth and nothing more.
On your second issue, if scientists first assume design, and the apply the scientific method to the evidence, the result is that design is rejected, because no evidence in support thereof exists.
Whereas, evidence of evolution does exist: (1) fossils lying in statistically predictable layers of soil, radioactively dated to match with their geologic location, said fossils found in simpler and simpler forms, (2) modern biological tests which have created new species of flies, by simulating natural geographic isolation, (3) bacterial studies demonstrating dna modifications under environmental stress, etc.
Unless you adopt the a-priori position that these things are all the product of the continuous intermeddling of some unseen supernatural force, then the scientific method observes nothing other than the change -- and the scientific name for that change is "evolution," because to call the change "design" implies the existence of some scientifically measurable extrinsic influence, which is not actually measured.
The assumption of design is thus the assumption that magic rules the universe. And, maybe it does, but magic is outside the purview of science, so to ask that scientists assume this position is to return science to the realm of alchemy.