Yes, it should do everything you've asked for. There's no reason why your DLNA devices should not work. If they don't, JRiver will most likely help adding support for it.
There are no need to have a 2008, 2003 server, home server or anything similar to run MC. It can just as easily be done by a client OS like Win7. I just mentioned a server because it has a role like a server, housing a 24/7 app like MC running as a server, and perhaps holds all the media. It's up to you if you want to use one of your clients as a server, or have a dedicated server for the media and/or MC server.
The way JRiver MC handles videos is that it uses bookmarks to figure out where you left of. It's a simple tag. This have to be synced to the server, then back to the other clients for you to be able to resume playback from other devices. You'll have to manually find and start the video you left watching in another room, and there is a slight delay for the bookmarks to be synced, but other than that it works like expected. The delay seems to be around 1-2 minutes from what I've seen. But I have not used this function much, and can not guarantee anything.
I'm sure UnRaid is great. But a cheap raid card, or even the built in HDD software raids can probably do almost the same job. And it can be run on windows. That leaves you with only one server and maintenance point. That's my point of view, and I know there are lots of opinions here. I love windows. It's the only thing I work with, and it does what I want. That might not be your case. So, you decide what suites you best as a server. I can not tell you otherwise.
If you do maintenance on a MC server, you CAN do this from a MC client on another Win machine. You'll just have to drop everything that can change something on the disk it self. Like renaming media files or adding or editing cover art. You'll have to log on to the server with Remote Desktop to do such things. The negative point here will be that you'll have to take over the session from a client PC running the "server". If you decide to use one of your HTPC's as a server.
That's all I got for not. Shoot again if you need something specific. I'd like to recommend testing it for your self though. It's not that hard to set up. It's mostly the Media Network settings that needs to be activated for it to start rolling.