Trying to find a way to say this correctly, but I'll probably fail...
I grew up on shlichus, in a place with pretty much nothing, and a family with extremely high standards. Having lived that life, and having many friends who have lived that life, and having met countless people in that life, I'd say that shluchim in general have a chezkas kashrus. That being said, the blanket statements above basically bestowing sainthood on every single one... let's just say it rubs me a little wrong. There have been more than a few instances of either lowered standards, or even varying levels of going OTD. To be clear, they are an extreme exception and a tiny percentage of shluchim. I'm just not digging the blind trust "cuz mesiras nefesh" line.
My take on the whole thing is to treat shluchim like you would anyone else's house. In the absence of hashgacha or firsthand personal knowledge, ask. Whether that means asking the shliach himself, or calling some people for references, or whatever it is you do before you eat at someone's house, do that. I'm not quite sure why it needs to be more complicated than that.