Helly Hanson Seattle-Firsthand Experience?

I don't own this jkt, but I did work in a Helly Hansen retail store until recently so I'm familar with it. The Seattle Packable Jacket (SPJ) is a 2 layer jkt so it's going to be heavier than a 2.5 layer jkt. As a total guess, I would put it at about 15oz. It's not a light jkt despite what the name implies.

The waterproofing is good. If it leaks, you can bring it back b/c it has a lifetime warranty and they will replace it (but you shouldn't need to do this). HH is really good for this. The breathability is so-so. It's normal for a PU membrane. The jkt packs into it's breast pocket which is a nice feature.

I'm not too worried about weight and more concerned with warmth while inactive. How cold would you be comfortable using this jacket in?

Well it's not insulated, so not very cold. It's just nylon with polyurethane and then mesh netting inside. It really is just a rain jkt. Maybe 45F is the lowest temp you could wear this in if you just had a baselayer on. For colder than that you'd want to add insulating layers. If you're looking at HH, I would look at jkts like the MIG which offer the same waterproofing and similar warmth, but you get a waterproof zipper and a much lighter jkt (~10oz?). The downside to the MIG is the lack of pit zips and there is no way to roll up the hood or pack it into inself. Kinda silly really since this is really the jkt that users would be most likely to want to pack.