The “immediately obvious they’re not living in reality” criteria is much dumber than you think. Someone can be delusional but because they can agree on basic facts, they’ll be released.
Speaking from personal experience, it’s often very difficult to get someone into treatment unless they’re either so far gone (unresponsive, or immediately obvious they’re not living in reality) that a doctor will sign off on it, or someone gets hurt and the justice system can act instead
Dc-Dc is pretty efficient, I wouldn’t worry about conversion after the initial 48v, but I would potentially worry about losses in poor quality home wiring on longer runs in bigger homes
Also if the pfsense router is where the WAN IP lives as might be the case in simpler setups where it is the wan router, it would just note that “hey thats me” and resolve unless there were specific rules preventing that traffic.
Unless specifically stated, generally fiber transceivers only run at the stated rate and would expect every channel to negotiate unless it had a 4x10 mode supported by both the transceiver and the device you put it in (which is what that DAC splitter is for), though once you split the lanes this way, a single pair would only ever run at 10g
Additionally the nature of CWDM on a single pair means the 10g side will see all four signals simultaneously as there are no filters on that side of the connection. QSFP+ runs like this because it’s cheaper to manufacture and a single transmit/receive pair is only necessary in much longer range transport which typically uses more specialized expensive hardware for better long range signal integrity.
In short: no, this won’t work and transceivers generally only ever run in one mode et once as configure by the switch/router they’re inserted in, and the modes you’re asking a qsfp+ to run in are mutually exclusive based on type (single lc pair cwdm at 40g vs 4x10g MPO/MTP with changeable modes). Fiber doesn’t have the same “negotiation” features that copper Ethernet does.
Number of users total isn’t a big deal, number of concurrent users is. If nobody’s accessing them often it’s not particularly hard on your hardware either