The LG L3 (MSM7225A) shipped with kernel 2.6.38. Moving it to 3.4 for Android 4.2.2 was a chain of silent reboots, nearly all caused by the modem.
The G2 Mini (D625) is a Tegra 4i, not a MediaTek LK phone: unlocking works differently, with its own low-level recovery mode and flashing tool.
The L3's stock kernel already had USB host. Missing: mounting the disk and a switch to force host mode, since the ID pin never reaches the CPU.
The LG L3 on kernel 3.4 reboots by itself, no panic, at 609–616 s of uptime; on 2.6.38 it doesn't. What I ruled out and what's still open.