The cat palm:

The pothos: (holding up better, but then, who can kill a pothos?)

The Sansevieria: (only one soldier fallen)

Welcome to Gardenweb!

It's reasonable to imagine the others drying in about a week (but I don't think so from looking at the size of the pots in relation to the size of the plants,) but it would be very unusual for any Sans to need water weekly while inside. Until the soil has dried significantly, potted plants don't need more water, and in some kinds of soils, some kinds of plants must dry completely or they can and probably will rot at the roots. Sans is one of these. The palm and PL would wilt if they get too dry, but still can't 'breathe' if roots are always soggy. If you're keeping the soil moist/soggy, letting them dry out much more than you have been should help. Roots need oxygen and moisture at the same time to be able to function.

Not my area of knowledge at all, but the fertilizer routine sounds like it could be enough to be toxic. It may have been night when you took these pics, but none of the plants look like they are near windows, so probably not enough light to do anything quickly, including using nutrients. If you haven't had these plants for long, there may be time-release fertilizer in the soil already that's not yet 'used up.'

The white spots on the palm, if not dust, could be concerning if some kind of pest. I would remove all of the yellow and brown leaves, as close to the trunk as possible. Then a pic of the whole plant could be helpful, if you feel like adding one. If it makes you feel better, which it should, those aren't branches on palm, they're compound leaves, so each branch is considered only one leaf. Losing the oldest one periodically is normal. If you remove the ones that aren't still green, (they can't turn green again once brown,) you can see the others better, and know how fast they are turning brown.

Doing too much of the things plants do need for people to do for them can be more harmful than good, like adding water and fertilizer. Does this stuff sound like it applies to you and your plants?

Ok, I'll work through some of those things, but the soil seemed fairly dry when I went to water-it wouldn't hold being made into a mud-ball or anything.

There was no fertilizer in the soil to begin with, because I made it from scratch, but the fertilizer ratio to water was so light that I might have misjudged or over poured the fertilizer liquid. I need to get a small measuring spoon.

There is a window right above the plants, but faces west, so I try to keep some blinds in place. In any case the amount of direct sun light was minimal and filtered.

Some kind of pest? One of the plants was getting some very small kind of gnat, but I tried to kill them. I haven't seen one for months, but I suppose they could be in some kind of winter hibernation.
In any case, thank you very much for your time and help.

Im not sure how "true" this is but i was told to touch the soil with your fingertips. If its 'sticks' to your fimgers, the plant has enough water. As the plant dries out you'll notice it more on top. Add a little water when the soil no linger sticks to your fingers but dont over do it. Ive actualy been able to bring one of mine back from the grave so to speak using this method and its doing great even though im not the best with plants. :)
good luck!

For plants that aren't too heavy for me, I lift them up to see if they need water. If they don't feel relatively much lighter, I know they're not dry yet.

HH, glad to share info, I'm sure you'll do the same when an opportunity presents itself! The light you describe is probably fine for the first 3 plants, but the Sans can be much more vigorous with more light and, after being acclimated, can handle hours of direct sun outside. Not that you're required to have your plant growing as fast as possible, that's completely up to you, but knowing it's not its' happiest in a darker spot is helpful to some people who would like to see more 'action' from Sans.

For more in-depth reading, this and this post have excellently helpful info.