So it's kind of like
"With a feeling that no matter what happens, it won't mess up,
a feeling of certainty, I understand you"

?

That's kind of altering the こぼれ落ちない a little bit from won't overflow/scatter, but would that work? I just feel like it'd be best to approximate something than just drop the line entirely. Since Sawao wrote it and it's therefore part of the lyrics, dropping it would make the translation incomplete.

maybe

"like a [bucket,bottle,glass etc] that won't spill no matter what happens,
I truly understand you"

?

That's a bit more literal, but I don't think that "tashika na kimochi de wakaru" should be "truly understands," personally. It's literally "understand with a sure feeling," so I'd rather convey that, since the two are separate things and I want to keep this as literal as possible while making it make as much sense as possible in English.

I'm not sure if I'm right on this, but it feels like he's using both phrases to modify "kimochi," like
何が起きたってこぼれ落ちないような(気持ち、)
確かな気持ちでキミを分かってる

"With a feeling that it won't spill no matter what happens,
with a feeling of certainty, I understand you"

So to make that first sentence into something that we'd actually say in English, I figure it'd be okay to change it to

"With a feeling that it won't mess up no matter what,
with a feeling of certainty, I understand you"

Does that make sense to you, or is it a little bit of a stretch? I'm not content with kind of smudging/tossing out bits of the lyrics, even if the ideas are repeated a little.

yes I think you can go with that, but lyrics are full of figurative language; it can be interpreted differently by different people... thats why I made it more literal.

I see some other translation on this site, and I can't agree to a number of them.

Well, if there are any grammar/translation errors, please feel free to point them out. I'm still learning, and I make mistakes. Interpretation is interpretation, but errors should be fixed when possible.

Thanks for your help, btw.