string gauge, tension and neck relief

I've had several teles with 12's that I tuned down to Eb, D or even C# to get a reduced tension feel. You can also get a dual-load tele bridge (Wilkinson makes one, among others) and top-load your strings. That will reduce tension too.

As for your truss rod/neck relief...I highly doubt it's set "for good." That's the point of having an adjustable rod. You can change it, even years later.

 

1) It'll be close. Depending on exactly what set of strings you use. I use D'Adario XL .011-.049 on all my guitars, tuned down to Ed for my band, and I have a really high action for playing slide. I've done that enough years that I really don't notice it being particularly difficult to play for the most part. However, when I play my friend's guitars with 10's, I can tell a notable difference.

2) That shouldn't be truly permanent, and the truss rod should give you some straightening ability. It might be the neck has settled into a slight bend that will resist straightening if it's too much. You might need to make more than one adjustment, in small increments. But thick carved necks especially don't move that much anyway.

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D'addario has a string tension calculator. stringtensionpro.com You can set your scale length, string gauge and tuning to compare tension of different sets in various tunings.