I haven't heard of creatures pathing through above-fluid fortifications just like that. Completely submerged fortifications can be swum through by swimmers, and dodging can apparently both cause creatures to get stuck within fortifications and to tunnel (in the quantum mechanical sense) through walls.
However, up/down staircases, and probably down staircases as well, can "leak" diagonally downwards, i.e. if you dig your up/down staircase you'll see rock in the time beneath it, but if there is empty space to any side of it (don't know if corners are included) at least swimmers and flyers can pass through diagonally upwards (without even notifying you about the presence of a cavern/the magma sea).
I haven't heard about problems with fortifications and trees before, but given that trees cause all kinds of trouble, I wouldn't be totally surprised if they managed to get branches into fortifications that allowed creatures (such as dorfs) to pass through them.
I use a combination of two techniques to explore the caverns.
1. The main one is to dig an up/down staircase for as far as possible (i.e. I designate digging to the bottom of the embark). When I get the message that a cavern has been found I immediately order the building of an Up stair in the staircase to block access from beneath (Warm/wet stone require the same plugging due to possible diagonal access, which hitting SMR is tricky because the miner will just stop without there being any message, but there is still a risk of diagonal access, so that has to be plugged as well).
Once I have some view of the cavern I build an airlock (two drawbridges in a tunnel with space in between, typically 7 tiles, and normally 3 cage traps at the end towards the cavern (I leave one tile of wall unmined to remain safe), and a door at the other end (to attract building destroyers). When the airlock has been constructed with traps and drawbridges hooked up to levers I study the visible wildlife in the cavern, and when a benign species is present I breach the wall after having set up a civilian alert to stop the dorfs from rushing out into the cavern, and send my "militia" (i.e. the dorf I gave the spare axe to) on an exploration tour. As soon as I see something dangerous, the explorer retreats and the outer or inner airlock drawbridge is closed (depending on whether I want to catch the critter(s) in the cage traps or not). When the cavern has been explored I send out dorfs to seal the cavern entrances (i.e. the openings on the 4 sides) to secure the cavern while benign critters are about, and withdraw them with civilian alerts when the critters start to drift out (the number of e.g. elk birds starting to drop). The drawbridge is closed and the civ alert is dropped. Repeated until the cavern is secured. I typically build a mason's workshop by the drawbridge (on the safe side) to produce rock blocks for cavern securing, although I also use cavern tree logs.
The cavern entrances are either bricked up completely flush with the walls beside the entrance (with stairs to allow the dorfs to seal the entrance completely upwards), or build a wall one tile away from the entrance (joining the walls) with a drawbridge in it for future development into an airlock. If the entrance is higher than one z level I build a couple of ramps beside the wall to get up on top of it, and build a wall flush with the entrance up there. If it's higher than two levels I build up/down staircases on top of the wall to seal the entrance all the way up. Don't forget to build a floor or an Up staircase above the drawbridge, or you'll leave an opening there! Lately I've started adding Gremlin traps to the design as well.
2. When water or cavern geography blocks the explorer's ability to map the cavern I either dig tunnels inside it (to reach a visible part of the cavern), or dig additional exploratory staircases (without the benefit of a message to tell me when the wall is breached). Each such breach is immediately bricked up and plans are made for a more organized breach (see 1). This technique is also used to probe for the other caverns, i.e. I see where there is a path full of rock past the cavern and dig a horizontal tunnel above the top of the cavern to get to above that location, where I then dig a new staircase as far as it will go (with appropriate adjustments when looking for other "missing" caverns, such as finding the first one and then the third one).
BTW, the 3 spare spaces in the airlock are for later additions of menacing spike traps hooked up to a repeater for elimination of FBs.
BadLeo got an answer in while I typed. How you want to approach the caverns is a matter of play style. I'm a coward in that respect.