Partial charges are always advantageous to the battery, so charging at half charge will give you far more than double the number of charges (about four times, actually). That advantage increases the shallower the charge, but there's no point in going overboard. Most of the stress from charging is near the top end of the charge--it'd be smarter to cut it off a little early instead.
That having been said, if you need to you shouldn't be hesitant to discharge it fully. Batteries will be good for at least 300 charges that way, drained and recharged fully, with our mods typically being a bit better than that because the cutoff is before the battery is empty.
Operation as a passthrough can impact battery lifespan...but it depends. Keeping a battery at the top of the range and constantly recharging isn't the best thing for it, so that can be an issue. It's better to allow the battery to relax and the voltage to drop, and then recharge at a reasonable level of discharge.
For the most part, I can't say I notice much reduced lifespan with my passthroughs, but that does vary. Some seem to really dislike it, others don't.
