Sparkling and avoiding bottle bombs

In general, to answer your first question you are stuck with a sweet still mead or a dry sparkling one. Sweet sparklers are possible but involve more work and/or luck. Methode Champagnoise is one way to get a sweet sparkler, if you dose the carbonated mead with sugar at the end (as is typical). There was talk about using dialysis tubing to help get the yeast out using this basic idea but skipping the riddling. Kegging or other means of force carbonating is the only other reliable and safe way to do it.

Also, 71B is not a great yeast for bottle priming. It tends to leave flavors that are not so tasty with prolonged exposure to the lees. In the future you might consider EC-1118 or similar yeasts (names like Epernay, Prise de Mousse, Champange, etc) for sparkling batches.

For bottle priming, I like to use fun tools like this one. Obviously it's meant for beer, but bubbles are bubbles. Most beers have about 1.5-2.5 volumes of CO2 in them. I don't like to go higher than about 3-3.5 in a beer bottle; at that point you are getting close to a reasonable expectation of the burst pressure (with perhaps a safety factor of 2 if your bottle is in tip-top shape). Champagnes are pressurized at a much higher level, but beer bottles won't hold that much. If you want to go higher, you need to use sparkling wine bottles--the big thick ones with a nice sturdy punt (dent) on the bottom.

Some things to notice about the calculator: There is no option for honey, but you can calculate a honey addition based on the corn sugar option, which is basically the same as honey (sugar-wise) except 100% sugar. Honey is roughly 80% sugar so take the number you get and add 25% (by weight). The other thing to be careful about is the "residual CO2" calculation. This assumes that the liquid is still saturated (or nearly so) with CO2. If your mead has been sitting around for awhile, it will not be holding that much gas anymore. If you're not sure how much gas is left, err on the side of "some" and you can make adjustments in the future. I find that a few weeks of sitting around means I really should be adding enough sugar for an extra 0.3ish volumes compared to bottling right after fermentation is complete, based on this calculator.

If you do prime for a high pressure (be it 3.5 in a beer bottle or 5ish in a sparkling wine bottle), take the precaution of storing the bottles somewhere that they can explode safely if necessary. A tupperware container with a lid is a good choice. I've never had a bottle blow up on me, but I sure don't want to get hit with shards when it eventually does happen. (and it will, it's all a matter of statistics.)

As Akueck mentioned, you probably don't want to try making a sparkling mead with the yeast you've used. It can cause off flavors with autolysis. Using a Champagne yeast with future batches will give you better results.

As for trying to make a semi-sweet sparkling mead - you cannot do it without serious risk of bottle bombs unless you force carbonate it in a keg. You can naturally carbonate it in PET plastic bottles which will become hard as the CO2 pressure builds, and will not shatter if the pressure is too much.

Attempting to make a sweet sparkler is to invite flying glass shards!

Bonne Chance!
Medsen

This may be heresy but how about making a dry sparkler, and adding a splash of simple syrup when you drink it?

Definitely not heresy, that is a great idea. You can make all sorts of flavored syrups and end up with 20 different meads from one batch. Sounds fun!

I don't have much experience, but I think the issue with bottle bombs is more serious injury/property damage/death than mess!

You can always mix your mead with sparkling water for a fizzy "mead cooler" type drink. Or you can buy CO2 injector caps that fit on plastic soda bottles and squirt some gas in right before serving. (Plastic bottles are not great for long-term aging but are easy to fill half an hour before you want to pour a glass.)

I've never noticed bubbles forming in bottles while they carbonated. Not a lot of headspace to fill, so the CO2 tends to stay in solution until you open it. As you mention, sediment could indicate a lot of things so it is not a reliable indicator. I think the best way to judge is to open a bottle periodically. If they suddenly start fizzing, you can bet that something is going on.

If you use the recommended dosage and sanitize your bottles & equipment, you should be fine.

I haven't had any bottles blow up on me (yet) but I don't think there is a "safe" time limit. You could have a bottle that is at pressures high enough to break the bottle but it won't break until it is moved or jostled.

That being said, unless you notice bottles with bubbles in them you should be ok. If your bottles are clean then they should all behave in roughly the same way, carbonation-wise. I have never heard of folks seeing a single bottle out of a batch explode while the rest were still, but I have heard of many bottles out of a batch blowing up while some do not, but are nonetheless very carbonated. Bottle bombs are not the sort of thing that sneaks up on you. Either the whole batch is primed to go or they will all be fine. (dirty bottles are the exception but you shouldn't have those, right? )

Store them in the tub with the towels for now, see how it goes. I am 99% confident it will turn out great. (-1% for "sh!t happens" )

The last batch that I had referment occurred after being sorbated and sulfited and backsweetened. Everything was fine at first, but at about 4 months, the bottles became fizzy. I did not have any bottle bombs or displaced cork (luck me ). While it is highly unlikely to have a bottle bomb after sorbate/sulfite, keeping it in a plastic bin is probably a good idea. Being safety conscious is always wise.

It is worth keeping in mind that Brother Adam of Buckfast Abbey noted that in his sack strength meads which were not stabilized, he would see evidence of refermentation for as much as 2 years during the warmer temps of summer.

Folks have indeed tried using things like Sweet N Low and Splenda as sweeteners for dry meads in the past. I've actually tasted several of these attempts over the course of some years, and I have to say that none of them were particularly successful, and you'd probably find them not to your taste unless you really like "diet" soft drinks.

The one relatively new product that I have not tasted in a mead is the sweetener based on Stevia plant extract. It is being sold here under the trade name "Truvia," in case you or anyone else wants to try to make a sweet mead with it.

I've personally given up on trying to find an artificially backsweetened mead that I like.

What about lactose? Beer yeast at least won't ferment this stuff.

Good suggestion! Lactose is in fact used by some folks to enhance the apparent sweetness of meads, as well as beers. But in my experience, since lactose doesn't taste all that sweet, you have to add lots (measured in pounds) to have much impact on the sweetness of your batch. Still, if you haven't used it before it is worth a try to see if you like it. ;D