if I can plug both lines to the router at the same time with no
If the ports on the router are actual routed ports, you might be able to configure one as active and the other as "backup" interface, if the given vendor supports something like that. Still, chances are that this is not the case. Alternative: LACP between router and switch (see below). Might not be supported on that router.
If however, that router's LAN ports are actually ports of a built-in 4 port switch (and they are on most consumer-grade routers - which would make it off-topic for this site, anyway) and if neither switch nor router-integrated-switch support one variety of a spanning-tree protocol (which consumer-grade "unmanaged" hardware seldom do), you'll just build yourself a 300m packet loop racetrack. It will melt under self-generated/multiplied overload within something like 5 seconds after you hook up the first end device to the switch and it broadcasts its first ARP resolution request into the subnet.
Ethernet must have a star/tree/cascaded-star topology (at least logically), so parallel links, rings and loops must be detected and broken up. There's two main features for that:
- Spanning Tree Protocols (802.1d, 802.1w, 802.1s, Cisco (Rapid-)PVST+ and probably some more) detect a loop and put one end of one link into blocking state.
- Link Aggregation (802.3ad, LACP) merges two parallel links (of identical technical nature and speed) into one logical link.
Link-aggeration between a Powerline stretch and a Gigabit link would be asking for trouble, what with the large difference in speed and RTT you'll experience.
But the most fundamental problem of the intended setup might be this:
150m is 50% over max length for Cat5e (or better) twisted pair cabling with the usual 8P8C connectors (vulgo: RJ45). You'll be lucky if you can get a usable signal at the other end; with some trickery (like clocking down to 10Mbit/s and by fixing to full duplex mode from both ends), you might get away with it. But probably not at Gigabit speeds.