Whoopsie, my bad. Still, even with pppoe-wan at 1300, the connection doesn't work, traceroute has similar gaps and the HE tunnel endpoint must be manually updated.
I ran a different test today: I shut down the HE tunnel and the guest side of the switch, then I reconfigured the wan to enable IPV6 (still with MTU 1300 for IPV4) and tried the traceroute test: from the router, it looks "patchy" just like earlier tests.
From the desktop, I can ping openwrt.org both IPV4 and IPV6 with reasonable times but a simple apt-get would fail on some sources and ipv6-test fails to load completely. The ISP still says "reachability is only guaranteed for a few institutional sites", so it's obvious they don't give a flying f* and I should just give up ;(
Thanks to those who helped 
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