A/B Testing is a way of comparing two versions of a web page and see which one is performing better. The version page can varied in title, headline, button or a page with different lay out and design. After creating 2 versions of the page, half of your traffic is shown the original version of the page and half are shown the modified version of the page. As visitors are shown both the versions, their engagement with each experience is measured and collected in Google Analytics dashboard and analyzed. From the data they provide, you can conclude on which page is performing better. You can read the manual on how to do A/B testing in Google Analytics to get better understanding.
All plugins that you mentioned here are good for SEO. Yoast SEO is one of the best plugins out there. We use it on all our sites and it’s one we always recommend. This plugin lets you to perform many onsite technical optimizations to improve the SEO of your website. It has a lot of features that make it all-in-one plugin.
Features: • Keyword Optimization • Complete SEO Page Analysis • Readability Checker • Full-control Over Breadcrumbs • Advanced XML sitemaps • Duplicate Content Alert • Technical Stuff in Background • Internal Linking Suggestion
Profile backlinks valuable method, but only when you’re looking for change rather than link strength. But you need to not just create a profile & posted the link, you should be active on that profile. Within 2-3 months the profile link will be worth something. Profile links can be very valuable if it is on a quality website.