1. It is right to keep your muscle tense and to exaggerate each sound while practicing. But when you over exaggerate too much, you could miss the easy parts while pronounce the harder phonetics correctly.
2. Trusting your eyes instead of your ears is basically right, especially when you are correcting your pronunciation of each single phonetic, but it does not suffice to ignoring praticing your capability of 'listening'. If you can't hear accurately, how can you mimick anything accurately? I'd train both aspects simultaneously so that each can help the other and in the end they (my ears, tongue, and eyes) together will make me accent free.
You are doing pretty good. I am just being picky on you since I have higher expectation on you. Wish I could be as strong and persistent as you.
Happy holidays!