I’ve done the Sparklan card swap and tested another card in the modem slot. From these and some other tinkering, I’d say there’s nothing special about the slots as per voltages or like that. You are safe to test. The first test is, does L5 recognize the card and offer wifi, as the supported linux versions list seems to include past and present Debians. If it doesn’t, lsusb. If it’s not there, my condolences.
Slots and cards follow certain standards and they have tolerances, so no big surprises should be expected - no magic smoke events. A slight extra voltage that these slots may have is not that serious (for testing at least) but if they offer unexpected undervoltage, that may prevent something from working. More likely than that, some connector or keying isn’t used, which you can do nothing about. I once came across one card that was the right size and keying but the used connection standard for data/commands wasn’t the one that L5 uses on those. My take is, there are no absolute guarantees, but the most likely way to actually do harm is the disconnection/connection of antenna wires (to wires and/or cards), that you already listed.
The card is A+E keyed. The old cards and slot are E keyed, but that shouldn’t be a problem, if my memory serves how those are meant to function (unless L5 needs the missing contacts for something).
Size seems small enough and smaller than normal. Interestingly, as BT has been left off, it’s not as wide as normal card and some of the pins from the far end seem to be missing. This may make seating the card a bit easier but also finicky. Make sure that contacts in the slot align right and not at an angle (which could short it). The keying gap helps with alignment though. Let’s just hope nothing of interest to L5 in those.
Post pics in any case when you do the test. It’s always a learning experience…