1. You need to read the full context of this sentence. Check the official guidance here: paragraph S3.9 of
https://assets.publishing.service.gov.u ... -02-22.pdf
2. a
3. Ask the employer to add sponsor note before you submit the application. While waiting for the visa decision, if you suspect there would be further delay of more than 28 days, ask the employer to use SMS to make another amendment.
4. No, but submit your application no later than 28 Apr
5. It's used to calculate how much IHS you need to pay and how early you can apply for the visa.
DISCLAIMER: Advice given is based on my past experience and/or my interpretation of Immigration Rules and UKVI documents.