For my seamless pattern, I duplicated the background color, then resized it to the canvas size and sent it to the back of the design. That's how I got rid of the seamless pattern.

I don't understand your explanation / question : can you tell us more ?

You have issues with svg patterns showing split lines between the "tiles" ? 

I think I know what you mean. Unfortunately that's how the vectors work. Even Adobe Illustrator creates such lines on the edges with 2 shapes next to each other.

As you wrote – to avoid that you need to add the piece of the same part of a pattern under the ones that are at the edges. The same with background. I think it is the best to create transparent patterns and when you need to put it on a desing, you just create a layer/object with needed color, so you don't need to bother with duplicating the background.

Screenshots for visualisation:

There is also a template for seamless patterns. I don't know how it works, but it doesn't seem to create the lines.

To clear some things up. 

If you convert an object to a pattern, that should render without a gap.

When applied as a fill to a selected object, and the handles adjusted -to see some repeats, where the gaps are supposed to appear.

The core of the rendering issue between two object sharing the same exact geometry is anti-aliasing, and how the semi-transparent pixels are composited atop eachoter. 

Anti-aliasing at an object's edge is supposed to smooth things out. So geometrically only a fraction falls onto a pixel to be displayed, and thus the alpha channel/opacity is reduced accordingly. 

When objects are next to eachother, their coverage adds up to 1. Together, they cover the area full opaque. 

However, the renderer doesn't know that. The two objects are treated separately. Like, one would be rendered into a 70% opaque pixel at the edge, the other at 30%.

Those values would add up to 100%. But they aren't. 

They are composited atop eachother. And if if you look at an object with a 70% opacity another one with a 30% opacity, you will still see the background. Close to 10%. 

So the issue is twofold, and it has to do with the renderer -every vectorgraphic application has this gap issue.

By the nature of it, it's only at the rendering level, within an 1 px radius.

Illustrator solves this by having a rendering mode which iuses oversampling. Rendering at a higher resolution and scaling it down to screen level. 

Can do the same if  you are aiming for a raster output. Export at a high resolution, resize it at post (in gimp for example). 

Anti-aliasing affects only a 1 px radius at any resolution. If you scale that down, it won't be noticeable. 

Other option? Make sure there are no object sharing the same geometry next to eachother. Overlap when possible. 

Adjust the zoom level so the anti-aliasing doesn't smudge the edges. That can be achieved if the coordinates are pixel-fitted and all the edges are horizontal/vertical. And you render at a certain zoom level/resolution. 

Get rid of anti-aliasing in some way. Like duplicating the objects atop eachother/themselves will result an image rendering closer to aliased. 

Like instead of 70% and 30% resulting in 9% background showing through, if you duplicate the objects 3 times instead the background would only show through at about  0.9%.

Also  using filters you can achieve a rendering where the alpha channels add up to 100% coverage, although it's rather an unpractical solution.

There were a few topics related to the same exact issue, but felt easier to answer it again than to  look up the previous topics.

Others may klnow better.