The ratio and tincture coverage challenge is difficult for many many herbalists. Quite often you will come across the issue of not being able to submerge all of your herb matter under the alcohol at stronger ratios. It is usually much more common for light leafy or floral parts and less so for roots. You are doing just what I would recommend really, by blending the herb as fine as possible you can create a more compact marc. Without having machinery for extraction or using an entirely different method, this is about all you can do. However, saying that, if you would really like to try to make your tinctures stronger, you could do a double extraction with new plant material. E.G. first do the full infusion of your 1:10 motherwort, then strain and add the equal amount of fresh motherwort for a second infusion to the menstruum (tincture), to create something close to a 1:5.
I am a little surprised that your dandelion tincture was the same. often roots are much better since they are heavier. Did you use fresh root? Did you grind this too? or chop it? In the case of dandelion root, I usually chop it into small pieces before maceration. But if you have tried all of this and it still isn’t submerging, you could try my above method of a double extraction with fresh herb.
In some tinctures it can be near to impossible to keep some of the material from browning a little. Try your hardest to avoid this, by regular shaking or inversion of the tincture and covering the plant matter sufficiently. If you do get some browning, it can still be okay, as long as it smells, tastes and looks alright and there is no evidence of mould or slimy substances. Saying this however, you do want to keep a closer eye on fresh plant tinctures. Since the plants contain water the alcohol is diluted, the preservative and anti-microbial strength of the alcohol will also be weaker.
For the second question: As an overall rule, the more you can chop or blend your plant matter (increasing surface area), the stronger your extraction will be. So I usually at least break up, chop or blend all of the plants that I tincture.
I hope that this is of some help.
Wishing you great luck with your tincture making. It sounds like you are creating all kinds of wonderful magic.