Hi there. Looks like you have some good options. I would like to give my two cents.
I hate swapping lenses in the field. It's inconvenient, and can expose the guts of the camera to dust and the elements.
I used to run with a kit of the 18-55 WR and the DA 55-300. Lens swaps occurred frequently. This resulted in a ton of dust on my sensor. Not just bits of dust either, but huge blobs shaped like hairs that are quite visible in my pictures. After a particularly bad incident like that, I ended up getting an 18-250 to replace the other two lenses and I haven't been happier since (both my 18-55 and 55-300 have been sold off since).
As for low-light, the K5 rocks in this category. Unless you are going caving, I think you will be fine with your Sigma 28 2.8 for low-light situations. Also, 28mm is a perfect normal length for interior shots inside of restaurants and other places like that. My benchmark for this is if you can sit down across from someone at a restaurant table and still get a comfortable shot of them with their plate of food. 28mm is perfect for this. 35mm is probably the upper limit for that sort of thing.
Based on your listed lenses above, I would run with a kit of:
Sigma 18-200 (outdoor/daylight)
Sigma 28 2.8 (indoor/low-light)
(This is very similar to my own personal two lens kit of the DA 18-250 and the F 28 2.8. A minimalist combo that can do 90% of the things I need it to do).
And then any of those wide angle lenses you listed in your last post would be a fine compliment to round out the kit. Go for the 10-17 fisheye because it's cool!
Have fun on your trip.
PS. Don't worry too hard about the weather resistance of the lenses. Coming from someone who purchased a K30 specifically to take onto west coast adventures, unless you're going right underneath Naigra falls, don't worry about getting your lenses a bit damp. They can handle it. I don't own any WR lenses anymore.