Here are answers to your question as per my view point.
1. Anyone who is computer literate and is using some kinds of software for their work on PC or mobile devices can go for career in Software testing so you can also do it. But there are some essential qualities needed in a software tester like ability to point out malfunctions and mistakes in working of software, a questioning and defect finding mindset where an individual always questions the working of things in day to day life and always tries to find some defects in them. Since as software tester you will also be developing test cases also an ability to visualize different testing scenarios also holds valuable. Besides this some knowledge is required about the software which you are testing and also on the testing domain you are working on like you may be testing software for Banking, Telecom, Travel etc domains. In your case I feel it would be easy to start career in software testing in some commerce related familiar domains like Tax, Banking, Insurance etc since you are in a commerce field.
2. Now when you shift from senior auditor to QA your field and nature of work will change. When you switch to software testing you will be part of software industry and when you work in software QA field your prime job will be to find defects in software, prepare test cases, prepare QA documents like test plan, test process improvement documents, test strategy etc. If you do software testing on domain related to commerce like finance, banking etc then you will feel familiar with the domain but your work will be different. Even as auditor you might be using various commerce related software to do your daily work but here your intention is not finding defects in software but it is to get the output you are looking for. Now when you do software testing you will still be working on software but here your intention will change as now your prime aim would be to find defects in the software. Actually there are two types of software testing, White box testing and Black box testing. White box testing is testing of internal code of an application and requires knowledge of programming language and software architecture of the application while Black box testing is simply testing functionality of the application from end users perspective without any involvement in testing internal code and programming knowledge is not required. It means testing behavior of application and its expected result for different user scenarios. So in your case Black box testing will be easier to start with.
3. Future of software testing I would say is bright as any company which develops software will need it to be tested. It is proper software testing which determines that product is fit for customers to use and can be released in the market. Software which is not properly tested and released is likely to disappoint its customers and earn bad reviews. For you as a starter you can start with manual testing since that will be easy but remember that nowadays things are moving more and more towards automation testing. Pure manual testers are finding difficult to find job now while automation testers are being greatly valued. So keep in mind that after starting in manual testing you have to keep upgrading your skill and learn scripting language and tools for carrying out automated software testing for various applications. So you need to constantly upgrade yourself.
4, Salary wise also it is decent and if you are in good MNC you can get package of about 5 lacs per annum and if in average or low paying MNC then about 2.5–3 lacs per annum. Again this is a rough figure and salary will vary based on your skill and position you are hired.