"The rig will ONLY be hotter than the ambient temp during lights on. "

It's a good thought, but you quit thinking about it a little too soon:
It will be ~at the ambient during lights out.

Relax, condensation will not be a problem.

If you decide to sweat solder, I'd advise you to do it right.
Use plumber's solder, acid flux, and a torch.

If you decide to goop it instead.
There is an epoxy stop-leak available that has the look and feel of modeling clay.

I had a thought about your thin tubes.
Not sure how you plan to do the manifold, but I see no reason to bother spiraling them.
You could just slide them into a larger pipe for a counter-flow chiller.
Or, you could just buy a counter-flow chiller.
The are sold at brewing supply stores for chilling wort.

'twere me, I'd just spread them out and let them radiate the excess heat directly to the air.
I mean, it's only 90 watts, yah?
Water cooling, while cool, is total overkill here.

I do have a suggestion to simplify.
Take all those fitting back to the store.
Buy a roll of 3/4" soft copper that's used for supply lines.
It's very soft, with thick wall. Can be bent by hand.
Mark it off at intervals and use a vise to compress flat spots at those nodes.
Sand the flat spots and thermal glue the leds to 'em.
Then hook up your slow pump if you wish.

But, since you are already burning energy to produce the excess heat why not use that heat to move the water.
Google "thermal siphon".

Designed one of these for a 400 Watt unit.
Then realized that 400 Watts was overkill for my purposes and scaled it down to 150W.
That eliminated any need for the complexity and expense of water-cooling.
I get by on a cake pan with a single fan now.
Attachment 290405Attachment 290406

But, I really wanted to build my kick-ass, water-cooled, monster just for the coolness factor.
Hence my vicarious interest in yours.

So, how can I he'p?

Weeze