Question: When you say "best/fastest signal" and "prove that your product is something people are willing to pay for", are you speaking of proving to myself, like a bootstrapped small business? Or of proving to prospective investors?
Regarding the paid&free model, just thinking aloud... I suspect that the browser vendors are going to push me to use their extension store, so maybe I have to put the paid features as data/behavior on a server that the extension talks to. Or to send a paying customer an unlock code that flips a trivially-crackable switch in the free extension, but which at least helps keep honest users honest.
Maybe set up a server where people can pay and people can auth against later on to automate the payments and updating the users list.
You have the advantage of being able to start and potentially get something into the market without needing to take investor money and promise larger than life returns on the money you take. Use this time wisely to make an honest judgement of what you think this business could turn into, if it's just something that pays your rent then let it be that. If you grow like crazy and need investment money, only then pursue it.
Dealing with investors is a really big headache, so only accept money from them if you really think it will make a difference in being able to grow the business.
Yes you will almost certainly have to use one or many extension stores, you will also need to setup a secure way to handle subscriptions and give elevated access to users that pay.
Offering what you have to this tech company is probably the quickest way to get whatever it is that you're doing copied. Companies pretend to be interested in things they have no interest in buying or just want to get a head start on by having you show them in great detail how everything works all while telling you they are seriously interested.
The tech company will only seriously consider buying what you have to gain the userbase you have built with your product, not the technology itself in 99% of cases.
2. There is no way to stop this, especially for browser extensions. Making it more difficult to copy the source code will slow people down, but this is something you will have to deal with especially if you gain any traction and prove that your product is something people are willing to pay for.
You have an enormous advantage over these people if you are the first mover in this space however. Focus the majority of your time on creating a truly great experience for your users that will build loyalty towards your product instead of trying to play a never ending game of cat and mouse with people copying your idea.