It doesn't need to be fully watertight. The rate of water passing into the dry area only needs to be lower than the rate you can pump it out.
The article does talk about NASAMS, which is exactly what you describe. Its worth noting that a lot of the range is consumed accelerating the missile from stationary, gaining altitude, and flying through dense air. So, their range is much shorter than the same weapon when it is air launched.
Ran a sub 20min parkrun. Haven't been under 20min is about 5 years, so I'm rather pleased.
I'm not quite as heavy as you 86kg (189#), but I get relatively sweaty. My long runs are 90-120min, and half marathon is my preferred distance.
I think the extra weight does increase how much you sweat, but you can't stop it without going slower. I'd recommend a hat, to keep things out of your eyes.
I've never had chafing on my chest, but you could look at wearing a tighter shirt so there's less movement. If then shirt is already tight, try looser. Maybe a singlet could be a good option for you too.
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