I finally did the installation I have asked a bunch of questions about. It
involved putting some Ts in the main water line.

One of the joints had a tiny leak, maybe a cup an hour. Reheating it,
needless to say, didn't help; so I pulled it apart, recleaned, refluxed and
resoldered. It is fine now. But I have some questions for future
reference.

1) If I had ignored the problem (it would have drained into the sump anyhow)
would it have been stable, or would it erode over time?
(A couple years ago a friend soldered in my new water heater and one of the
joints has a tiny leak; maybe a cup a day. I wrapped a cloth around it and
it evaporated faster than it leaked. A year later I put in a pressure
regulator and it stopped. Apparently the joint was okay at 55PSI, but
leaked at 90PSI.)

2) I have some plastic stretch tape that claims it stops leaks. If I
applied it with the water off, would it have stopped the leak; had I not
been able to take it apart? If not, would any product?

Thanks

I don't plan on doing anymore plumbing in the new future. Oh, not quite
true; I have to replace the kitchen faucet. Hmm.