There are also the legal restrictions for the Trump Administration against keeping regular Army soldiers on the southern border for long (”emergency,” etc.). The Democrats and some Republicans will put up a really ugly fight against that. We can’t afford to lose Republican cohesion, which loss would allow the Democrats to win elections.
I think the Trump Administration is doing the best that they can.
That is a huge myth.
Those who invoke Posse Comitatus as a reason not to defend our border do so because they're looking for an excuse not to defend the border at all. Their claims are based on a complete distortion of what Posse Comitatus means (i.e. not using the military as a police force). It's just as specious an argument as the idiotic refrain we keep on hearing about illegal immigrants: "Well, we can't deport them all [so why bother deporting any of them]."