Vance,
Mead is definitely a lesson in patience. My biggest (and hardest) learning experience with mead is to let is be! Far to often I have done more harm than good by fiddling with it. You should really only rack it after the initial fermentation phase if it's been 3 months since the last racking and there is a sediment (you can go longer without much sediment, but I still rack every 6 months at least to avoid excessive autolysis and enjoy a taste test).
If you are at 18.5% it's a safe bet that you are not going to ferment any more (though careful adding in honey/water to top off - this will sweeten your wine, but also dilute the abv and could re-start fermentation - don't worry about your headspace too much, use a little metabisulfate or some sanitized marbles if you are really nervous about it, but I've never had issues with large headspace in meads before)
That said, with an airlock on the carboy you shouldn't have "evaporation" of the mead (are you taking samples? I loose many meads to this type of "evaporation"). If you have a small batch in the fridge, you should treat it like the large batch - putting it in either an air-tight container or with an airlock.
Clarifying is the bain of the vain in some cases - taste is a better indicator, and really what matters the most. I have had meads take up to 18-24 months before they cleared naturally, then again I have bottled great tasting cloudy meads with great results as well. However, once the others did finally clear they were beautiful and tasted amazing.
You should think that it will take at least 9-15 months before your mead is good enough to bottle (once bottled you can't change anything). I have sped this up with clarifiers like bentonite and sparkoloid (I wouldn't recommend any others), but you should try using a quarter of the recommended dosage and waiting another month or more - I used the full strength of bentonite recommended once and while I got a very clear mead, it did strip some of the flavors and made the alochol more pronounced. Less is more, because you can always add more but never take away...
Since you're working with a high alochol mead you shouldn't worry too much about it's safety too much - it is made to age, will age extremely well, and (so long as you have an airlock) should be fine in the carboy with some headspace. It will probably benefit from small amounts of potassium metabisulfate every other time you rack (to avoid oxidation), but it's not necessarily critical if you don't want to add chemicals - again, you can go pretty light on this too.
I made an 18% carrot blossom mead that tasted horrible for the first 1.5 years, then cleared on it's own and tasted amazing. Time will tell. Oak might be a good addition (oak cubes for 2-4 weeks, to taste), I'd recommend French or Hungarian oak.
Hope that helps!
Joshua
p.s. The white foam on top is probably offgassing bubbles, honey is a thick substance and it is normal to see this on top.