These sorts of generic questions are very hard to answer because it depends on a lot of factors, such as the nature of the files - small, large - you'll be using, the concurrency, speed of access, what else you're running on the box, etc. With encryption you already have a problem because I'm pretty sure that processor does not have AES acceleration support, so you are doing the whole shooting match (running the environment/OS, RAID checksumming and encryption) all on the CPU - and it only has 2C/2T. So everything should work fine in your base state in paragraph 2, and then I would expect everything to go right out the window performance-wise as soon as you encrypt and/or try something disk intensive - multiple file reads and writes simultaneously, etc.
So, in order :
1. Not optimal given there are a reasonable number of reads and writes to the boot volume, although there is a plugin to reduce the wear on a USB stick when using it as an OMV boot drive if you must. Instead though why not save yourself the pain and buy a small SSD, 128GB or thereabouts, which is going to be the price of a nice dinner at most?
2. I can't see 8GB being an issue unless you run something like ZFS + a bunch of containers. I run ZFS over a ~12TB array and around 30 containers - some fairly chunky - and it uses about 70% of 16GB of RAM (but under 10% of CPU unless I'm running a large search or index workload). As long as you don't use encryption (point 3) + virtualise all the thingz I can't see the CPU struggling.
3. I don't think encryption is a great idea until you move to a stronger CPU. Do you really need encryption? The likelihood somebody wanders into your house and steals your drives so as to be able to read stuff off a whole disk are low, surely? Why not just encrypt individual files or a special directory if it's really needed? And if your powerline adapters are anything like the ones I've used, you will be lucky to get 20-25MB/s, even on the "1GBit" ones. This has nothing to do with OMV, you'll get the same off FreeNAS. Unless you mean 100Mbit/s, in which case you should get that (~8-10MB/s) unless your house power wiring has issues. Personally I use a pair of Ubiquiti Nanostation Loco m5s to link the two floors, replacing a powerline solution. I get 30-40MB/s and it is much more stable and predictable than powerline.
After years of using things because they were interesting technically, lately I've moved firmly into using things because they work. I don't want running my home infra to feel like a job - OMV works OOTB and it's dependable, there's no need to make a rod for your own back in this area or others. (In a similar vein I would suggest biting the bullet and simplifying your storage with some larger drives, buying a second hand HP Microserver Gen 8 for iLO access and speedier CPU options, spending ~$80 on 8GB more RAM and an SSD boot drive, etc. but I totally understand that financial constraints can prevent this.)
