I'm curious what your main reasoning behind having a NAS and a WHS? From my understanding the Synology has a iTunes service which, from what I've read but have no experience with, should be able to provide iTunes service to your AppleTVs/iPhone/iPads. What other devices are you planning on using? You mentioned for heavy steaming, how many simultaneous devices are you planning to have running? You also mentioned using iHomeServer (which runs your iTunes as a service and now provides DNLA services) why not use that to provide for both iDevices and other streaming devices?
I would additionally ask the following questions:
1. Will your WHS be a standalone server or used as a desktop?
2. What devices will need access to media? Is there a common media format between the devices?
3. Why the need for ISO and MKVs? Do you know those are not valid formats for iTunes? Is that why you mentioned Plex, if so that will only be used for non-iDevices? Why not encode with handbrake into a format that all devices can use?
4. Are you running any additional applications or services? example be VMs, handbrake, SQL, etc.
5. What are you data requirements? Total today and how much growth expected over the next year? Can you NAS handle that or will you need to upgrade drives or model within the next year or two? Will your current WHS be able to provide for all your data needs?
I ask all those questions because you seem to be caught between using a NAS and a WHS, both are valid solutions but why not focus on one instead of having multiple streaming devices? How are you going to organize media between the two devices?
A single solid WHS should be able to handle all your needs. Leveraging DrivePool you can have duplicate or even triple copies of data, provide a platform for hosting user shares, leverage a offsite cloud backup software solution and a streaming platform. You don't need a lot of CPU power to stream to multiple devices, I was using a old P4 quad to stream to multiple AppleTVs, iPhone, iPads and computers with no performance issues. But you need to determine what else your WHS will be used for.
And regarding crashplan backups, depends on how much data you have, your outgoing bandwidth and timeframe to backup you are willing to accept, backblaze will likely be similar to crashplan. Just to give you a little background I have been using Crashplan for about 3.5yrs and currently have 14TB backed up. I spent roughly the first 6months backing up all my data. I've had to do a restore of about 10-12gb a couple of years into it, but I didn't have any issues doing a restore over a couple of days. The key is creating backup sets and organizing your data and setting priority. I currently have 5 backup sets created and in order of priority. This way the most important items are always backed up first and cascaded based on relevance to me and acceptable risk.
1. User Data & Pictures (this includes key user profile data like documents, emails, financial data, work documents, iOS backups and pictures)
2. Misc stuff (I have all my car photos, tune software, basically random but important items to me)
3. Software (basically any online purchased software, copies of software CDs, license keys, etc)
4. VMs (I keep quarterly backups of VMs. You will not want to have an active VM being backed up as the amount of changes will cascade and never catch up, if the VM if important consider every other day, week, or monthly backups)
5. Media (this is all my audio books, ebooks, music, movies, TV shows)
And to add an additional layer I have all my important used documents and pictures stored on my desktop(used daily) which gives me local access. These items are scripted to copy to my WHS weekly into my user share, that share is part of my User Data & Pictures backup set. So now I have 3 copies saved, 2 local and 1 offsite. Plus this saves me of only needing a single Crashplan backup license but still having data being backed up from multiple computers going offsite. So I would suggest considering setting up something similar to leverage your local copies and hopefully a single repository to be used to backup to Backblaze.
These are just some ideas to consider. Its taken me a handful of years to get organized and have my data coordinated and streamlined. If you are interested in my hardware you can see my post under the DrivePool > Hardware > My Rack Server post.