I'm not sure that your first argument holds much water: an independent candidate can be as appealing as they like but ultimately is constrained to appealing locally rather than nationally. I think such famous independents as Martin Bell or Dr Richard Taylor were successful partly because they were able to appeal to local issues (in Bell's case, admittedly, he targeted a politician who represented a wider issue), but can't do much nationally.
I take your point on the second argument, of course. As I say, I was being more than a little pedantic, but MPs should be allowed to be flexible and while there is a national manifesto I am sure that many or most also focus on local issues. It's been suggested that more than a few Labour MPs in the 2017 election were disavowing Corbyn and trying to fight locally, and still did well -- so did they win despite, or because of, the Manifesto? Hard to say.
All this does is reinforce my feeling that FPTP is the wrong electoral system for our country, or indeed any one. My snap judgement is that I'd rather like to vote for the Independent Group, if I got a chance -- but, for the moment, I won't have that chance, because none of their members are in my constituency. Even if I later decide that they aren't worth voting for then it's still completely unacceptable that I don't even have that choice.