The super-foreshortened man example isn't appropriate for 3/4 view anyway. What you want it something in the middle between those two versions.

The original RPGM caracters are in an appropriate view, but they're also chibi, so if you try to reproduce their proportions just by shifting the perspective on your characters, you'll end up with something that doesn't match the 3/4 world.
Part of the issue though, is those other assets you're using aren't all 3/4 view either xP That clock, for example, is a travesty xP RPGM's default graphics are a bit thrown together and aren't all in the same projection and scale. So, you may want to create a few simple assets of your own to use as a guide for your characters to make sure it all works together.

Perhaps instead of diving into details like you are, play with silhouettes first. That will let you iterate with different views faster, as well as help you make sure the characters read well at a glance no matter what they're wearing. The way you're colouring/shading them is also working against the foreshortening you're trying to do, so working with silhouettes will help you get a good starting point for the colouring, which people can then critique separately.

Resolution mixing is when you have some parts of an image (or game, or whatever) at a different "zoom" than others. For example, the character is at 1x (1 pixel in the image = 1 pixel on the screen), but the helicopter looks like it's been zoomed in 3x or 4x before it was placed in the image with the character.
Keep all your assets in the same scale at 1x. If you want the zoomed in "blocky pixel" look, do it afterwards the entire resulting image, so that everything is zoomed in the same.

It's really cool that this is possible, but I can't help but see these as "bad pixel art". The textures and level of detail on these models don't lend themselves very well to this process, in my opinion - too much noise.

The pigs don't immediately read as pigs, and they're a bit hard to see. If you make the rimlight on them greyer, it'll look like red/pink compared to the green of the scene, which should make them look more like pigs.

The scene also doesn't feel very dramatic because there are so few pigs. Put pigs behind the characters, or show what it is they're defending (also in low detail, since the focus is on the three humans). I think you should also move the current pigs up, so that they're overlapping the green grass more, which will help them stand out a little more, as well as add drama to the scene since it'll make them closer to the humans.

I'd also change the characters' drop shadows to be shaped like their shadows rather than blobs. As I recall, that game has shadows shaped like the sprites, so taking a step "back" towards the most primitive type of shadow looks odd. Since the lighting here is from above, all you'd need to do is break up the shapes a little, it's not hard to make them look like cast shadows.

How big is this chest supposed to be? You gave a lot of useful details, but left out that very important bit :]