I'd guess that they ignore that problem and have baked the time it takes to get data from several tapes into the 3-4 hour estimate.

If you think about it, writes are more common than reads on average, so it's more efficient to just write to whatever tape is online and deal with the fragmentation problem on the read end, as opposed to queueing writes until the 'correct' tape can be brought online just save some time reading. Also in backup situations like this, it's more important to get the backup done in a timely manner.

That's true, but they're obviously using a multi-tier solution, with about a 90 day buffer before things go to tape:

  In addition, there is a pro-rated charge of $0.033 per gigabyte for items deleted prior to 90 days.

So it may just be feasible to organise writes so they end up together. But, yes, ultimately it is probably not worth it to do so.

The multiple-hour window could give you a lot of wiggle room here though. It's unlikely to take 3 hours to restore from a single tape, so even if you have to visit 2-3 tapes then you have plenty of time.

I'm sure that there is a general tiered storage platform (as mentioned above) which keeps some of the data online as well. That would let you run a "defrag" algorithm later if you find you need it.

Given it takes 3.5-4 hours to retrieve a backup before you can download it, probably not.