Old lava flows more likely to be flowed over again. Elevation does not matter much if the lava is all layed down flat.
Typical pioneer species depend on type of lava. On a'a it is ohia, but they are very slow growing and are going through a lot of disease problems lately. Pahoehoe lava is extremely hard stuff, and takes very long time to re establish plants on that.
My experience with lava flows here is that lava flows in bands; each flow creates a wall that directs subsequent flows to its side. Only when it's directed a large enough amount of lava through side paths (to the point where they're interfering with its ability to move downhill) does it then get the ability to flow over the original, more central paths. But I suppose it depends on your lava properties and topography. We tend to get really thick a'a flows here, dozens of meters near the edges and potentially hundreds near the craters (that said, pahoehoe happens - I've even got some old "fossilized" pahoehoe on my land - but is much rarer).
It's harder for me to make colonization comparisons because up here, the first thing to colonize - everywhere - is moss
Big thick blankets like pillows. It doesn't care whether it's pahoehoe or aa.
a real estate guy tried to convince me that the softer lave could simply be crushed down by heavy equipment and then you would bring in top soil. Would something like that be practical?
Ehhhh..... technically possible, but I'd think that would cost a fortune. I mean, that's how they build roads through it here, but they're not cheap. A'a is nasty stuff - hard to flatten, but very irregular so it's hard to fill too. Sometimes it's worse than others - I once walked on one flow front that would best be described as walking on a mix of broken vases and razor blades. It wears down with time, though - but it's always this chunky, hard, bouldery mess.
Also, we have plenty of active volcanoes but little production of lava. Ash is the big issue here.
Ironically we get that too here
Volcanoes that erupt through glaciers make ash even though they're basaltic, as the water makes it explosive. Our lava is also sometimes abnormally gassy (read about the Laki eruption for a
really bad example. And while we're overwhelmingly basaltic, we have a small proportion of our volcanoes that are predominantly andesitic / rhyolitic (the most famous being Hekla and Askja), so they're explosive ashy things similar to what you're used to. It takes some interesting geology to get that in the middle of an ocean; it's secondary rhyolite, caused by basaltic magma separating out over time into an mafic fraction and a felsic fraction
Hekla was thought in Europe in the Middle Ages to be a gate to Hell, and was often drawn in terrifying fashion on old maps; it had on several occasions created eruptions so ashy that they dusted the mainland.