Flash and Java makes packages that you can download and just push via group policy that don't require admin rights. It's not the normal package, you need to look for the network redistributable one.
Bad idea... pick one and use it. There's no reason to have two firewalls unless you have some sort of crazy DMZ, which I'm going to guess you don't. Otherwise you're just adding complexity.
I'll echo that you're over complicating it- the only way I see to do what you want to do is to buy a managed switch and have both routers connect to it as well as your "media pc" then have it direct traffic appropriately- which means about a $600-$900 outlay.
With what you currently have setup...
I used Dell Powerconnects at both of my last two jobs without issue.
One thing to keep in mind with them, is when you buy them, buy one more of the largest one than you need since it's cheaper just to have a spare laying around than to put the warranty on the rest you do buy.
Check out Crashplan, cheap, super reliable with good encryption on compression. I've migrated to it at the last two companies I worked at- it's a great product.