Answering your questions:
- According to the relevant ministry (BAMF, see https://www.bamf.de/DE/Themen/MigrationAufenthalt/ZuwandererDrittstaaten/Migrathek/BlaueKarteEU/blauekarteeu-node.html), the degree should be relevant for the job position. I'm not sure how far your particular degree is from IT one. IMHO, "fixing" it with "quick online course" will not be possible. Maybe one can get a bachelor or directly master degree in IT online within next years, if it is relevant.
- yes, the blue card salary borders (one generic, one for jobs with a lack of specialists) should be reviewed once per year. The salary border 2020 for jobs with a lack of specialists is 43,056 Euro per year brutto.
- you can find it on the web site of your local Ausländerbehörde
Besides Blue Card, there are at least 3 more types of residence permits one can get in such a situation:
You can check my answer about the difference between Blue Card and residence permits for the purpose of working for further information.
Without any kind of relevant education after school, from my personal understanding (I'm not a lawyer), even though the AufenthG §18a mentions vocational degree needed, somewhere deep in the law there was a regulation that 3 years of relevant work experience in IT might be counted instead. So if one has 3 years of work experience in IT, one can apply for residence permit with the purpose of work without any degree (this statement needs to be checked since it might be a subject to some law changes).
It is worth mentioning that the residence permits for the purpose of work will have some limitations against residence permit for the purpose of family union and vice versa. An example might be an employer lock for the first 2 years (if one has a residence permit for the purpose of work, including Blue Card, within first two years a separate application to Ausländerbehörde is needed to change the employer).
All in all, I would recommend you to discuss your situation with a lawyer (specialised on immigration law and with good reviews) in order to decide which type of residence permit you should apply for.