I hate to confirm your suspicions, but this looks exactly like a juvenile Sailfin Plec (Glyptopericthys gibbiceps), which are from the Amazon and therefore need tropical conditions. Also, in my experience these gorgeous little spotted things grow to be monsters of about two feet long!
They are hardy and will probably survive for a good while at room temperatures, but they certainly won't be happy in the long term, so I would suggest either take him back to the shop (wearing a suitably stern look!) or invest in a large tropical set-up.
Sorry - that's probably not what you wanted to hear!
Amphibians have permeable skin, so they can take in oxygen directly from the water as well as from air via the lungs. You'll probably find that your tank is so well oxygenated that they don't need to surface much at all to get extra oxygen from the air - it requires a lot of energy for the little blighters to reach the surface so they don'tdo it if they don't have to!
My personal preference is for just such a system - Berlin method with a shallow Aragonite base - it's more natural and as long as you only have a substrate of about an inch in depth, and add some benthic ceatures to turn over the sand and keep it from getting anoxic, it works really well. Remember to stock very slowly, as the live rock will take a while to build up a decent colony of nitrifying bacteria within it. Good Luck
All a venturi does is use the pressure of the water running through the filter to draw air into the tank - if your filter is located near the surface (as it should be) the venturi makes very little difference to oxygen levels in the water - all it does is make the tank look a bit prettier and keep you awake at night!
Where the fish are concerned, it's far more important for them to have a dark period of at least 8 hours than for them to have light during the day. This is so their metabolisms can have a rest period, otherwise they will become very stressed out. The plants in the tank won't be affected by a day without direct lighting. Either just leave the lights off until you get back, or invest in a plug-in timer (quite cheap from B & Q), which is what I've always done (being the lazy sod I am!)
When you say you replaced the strip thermometer with a "digital" one, do you mean an electronic one, or another type of "stick-on" one (which are also known as digital thermometers)? The stick on type are notorious for being affected by the room temperature, so in a centrally-heated house will usually read higher than the correct water temperature.
I've used Fluval 404's since they first came out, and love them! The Aquastop valve sometimes gets a bit of gravel inside it, which means it doesn't shut off the water flow as it should, which I suspect might be the case with yours. When it's working properly, you should be able to flip up the hinged plastic flange thingy and shut the water off completely (make sure you push the thing all the way up). If this doesn't work, and there's no gravel in it, then there might be a fault with it and I'd take it back to the shop where you bought it.
In my experience, these vacuums are useful if you have particularly messy fish, which produce a lot of solid waste, so you can remove this in between water changes, but the bag gets clogged very quickly, so it's a bit of a fiddly process. I'd recommend sticking to a normal gravel syphon to remove the solid waste, because you can then do this as you change water, which kills two birds with one stone, so to speak!
Common Plecs (the correct Latin name is Liposarcus multiradiatus for 90% of the Plecs sold in the country, by the way) originate from the Amazon basin, where water is soft and salt-free, so the addition of salt will be a stress factor, especially if it is kept in a coldwater environment. However, they are usually very robust, and will adapt very quickly to the different water conditions, as long as this is a short term change or is a very gradual change.
I wouldn't be too worried unless you begin to see a lot more algae in the tank than usual, or spot that the Plec's belly is getting hollow, which indicates that the addition of salt has stressed the fish enough to affect its appetite.