1) Read this webpage, and in particular the "Proof of work experience" section to learn what documentation is required for any work experience you claim.
In some instances, people who are not able to get employment/reference letters are able to provide sufficient alternative documentation (e.g. contracts, job letters, paystubs, tax documents, etc., letters from colleagues and/or customers). But it also requires a statement that you tried to obtain a letter from your employer, and they refused (along with some evidence, if you can - like an email where they refuse to provide the letter). There are no guarantees in this case, but many have succeeded.
2) The Minimum Entry Qualifications (MEQ) for the Federal Skilled Worker (FSW) Program include one (uninterrupted) year (52 weeks) of skilled work in a single NOC. Once you have met this one year requirement, gaps do not matter. The person who was refused for a gap must have been lacking a solid year of employment. It sounds like you meet this requirement.