Doug,
With down it is all about taking the proper precautions and having a strategy for keeping it dry under all conditions. The combination of Sierras and a full tent do really make your fear of it demonstrably something that is not reality-based. I think most of the staunchest supporters of synthetics in their proper context would agree with me on that one. If you still feel you need the safety blanket (no pun intended) for that one in a million case then bring along a 1 oz or so space blanket for the worst case scenario.
Glad you've begun to crack on that issue. The space alone you could save might allow you to drop down to the next smallest class of packs, for instance. There are not many replacement you could do that would have that kind of effect with zero reduction in comfort.
Here is a great 1 lb bag:
http://www.westernmountaineering.com/index.cfm?section=products&page=sleeping-bags&cat=extremelite-series
Most people find that the 35 deg rating is even somewhat conservative. I am a side sleeper and never had any issue with this one. Unfortunately it is pricey, but it will last as long as you will with proper care. Packs down to almost nothing. Think about getting the wider version if space to tumble is more your thing.
There are other examples too.
"The question then becomes will I save at least as much weight by reducing clothes/ sleeping gear as the penalty hit I take with a heavier tent."
This is an issue that has been dealt with here and elsewhere at some length. It is almost always by far the most weight efficient to go with a heavier down sleeping bag. That in and of itself is a no-brainer. The question is not total weight, but how far you are willing at the stage you are at to push this equation until other thing start to get compromised – like your arachnophobia. So you should be thinking about it in those terms. Get the lighter tent, and just bring a 20 degree bag (zpacks makes one that is less than 20 oz).
As to warmth of a tent, as a physicist I would hazard a guess that the warmth has very little to to with the 4-season vs. 1 season per se. A full enclosed shelter will minimize the convective heat loss. On the other hand this same air flow is what you want to keep the condensation down. Again context is everything. I find a tent like the black diamond firstlight does exactly the same thing with warmth for a pound less, and if you can afford it you can buy some ready made carbon fiber replacement poles and get it down close to 2 lbs. This is a bit heavy for UL, but seems perfect for you. However, again, context is everything. This is not a tent for conditions where there is a chance for a continuous 2-day downpour.
I'm wondering if the better solution to the dog issue isn't just to take with you a perfectly sized sheet of some tough (and cheap and replaceable) material such as tyvek to put *inside* the tent. That way you could decide on your tent based on its other features and not have the floor material be a make-or-break factor – kind of a dog-proof footprint for the inside of the tent.
Anyway, just more ideas to throw into the pot. The big three and clothes are %95 of it at the stage where you are at, and you should think in systems, as you have no doubt read here numerous times.