All this is pretty off-topic and possibly of interest to no one reading these forums but us. But, I'll asnwer your questions.
I'll spare you my resume, (unless you ask in private).
You are correct or close on all points; So I don't doubt your background or experiance.
A DLT does not cost quite as much as 10-20 300gb drives. They range from 1500 to alot more (10x's that for a good autoloader/library system). I think a 300gb drive is around 330.00 for the external model he is looking at. Used DLT drives can be had for as little as 700.00. So a more accurate range would have been 2-20x's as much.
Media cost is fairly high. But a fraction of the cost of the same size disk, which is his alternative. Single DLT tapes cost around 75.00, bulk orders can pull this down quite alot.
DLT's fail VERY VERY rarely. The drives do fail. But they usually come with very generous warrenties. The tapes can fail, but I have seen literally 100's if not 1000's of DLT tapes and never had a failure of this media type. DDS3 (now replaced with the bigger, better, etc DDS4) is slightly less reliable, but not a problem. I have had 7 yr old tapes that were poorly maintained (re-used, etc and then left in a desk drawer for many years) and had over 90% success rate with these too. Reliabilty is what tape drives are all about. You can not denigrate them for this. This is their strength. Their weaknesses are cost and the hassle of all the tapes having to be correctly cataloged, managed and stored.
I would disagree with you that SCSI is less valueable for small servers. Even the smallest server must support concurrency and no IDE drive supports this. But all SCSI drives made in the last 10 yrs do. They are faster as well. But most importantly they are more reliable. IDE drives are fairly fast (certainly fast enough for most uses), and very cost effective for all but the most demanding of tasks (ie: servers and very high end workstations, or A/V production). Also if a SCSI drive dies you will probably be able to (at a cost of 350 to 5000) be able to recover nearly 100% of the data. Being the cheapskate that I am, I move my home main system (not my home server) from SCSI to IDE. 4 months later my 3yr old kicked over and killed my external backup HD. 2 days later (while I was waiting for a new back up HD to arrive) my new internal HD up and died(not related to my son, just bad luck). I had alot of very valuable data that was no longer on any working HD. I called around and found the best data recovery service. They charged quite a bit, but were not able to recovery ANY (0.00%) of the data on my drive. I also recieved a very expensive lesson on IDE drives from the lab. I spoke at length to them about it and they told me that they almost never recover data from a catastophic drive failure if it is IDE and almost always do very well on SCSI. A bitter lesson to be sure. Especially since I had only a 4 months previously been strictly SCSI.
Raid write speed does vary with the factors you list. But even in a less then ideal setup without even a proper raid controller, even a SCSI software RAID will be significantly faster in write speed then a single drive. It is just how SCSI works. It's costly but cool.
Sure a fixed (meaning sealed) CD-R/DVD-R lasts longer then an unsealed one. And they are easier to handle and store then tapes. But a good optical jukebox will cost you as much as a DLT solution and is not more reliable or longer lasting then the DLT solution; it is mainly because it is more convient. If you take a huge stack of"fixed" cd-r's or DVD-r's and a plain old DLT drive (remember they hold far more data) and leave them in a metal filing cabinet for 7 years, which one has the higher proablity of failure during a restore? The optical platters. It takes many more of them and they decay even when sealed (ie: the difference between water proof and water resistent). I don't know of any shop that uses optical backups for enterprise solutions. There is a reason for that. Total cost is higher (much more media needed). Handling of so many media elements increases the probabilty of misplacing one, and of course their is the lack of longevity. Do you know of any IT shop that is using optical media for backups at above the workgroup level? Also, I don't know of anyone that takes the time to seal their media. Adds time, money and complexity to the process.
I don't think mirror drives are the solution. He already has RAID. This is the better solution, just not the cheapest one. I think using large low cost drives (either externally or in another low cost backup server) as his backup system is a viable alternative to the fairly costly tape method.