Best fit networking solution

So, this is a bit lengthy, but I've found many an answer on these forums before. I'm no network professional and I'm only modestly computer literate. I am trying to find the best fit network solution for my household, as the current setup fails at certain times of the year (I'll explain).
The current setup:
1 Ubee cable modem/4port and wifi router (bridged, I believe)
1 Linksys E2500 wireless router
Devices (most of the time):
1 PC connected via 100ft ethernet (had some issues accessing the webui of the router, though I understand this might be an issue of cable length)
1 vizio smart TV
2 Roku 2s
2 TWC HD-DVR boxes
1 PC connected via wifi
sometimes:
1 laptop connected via wifi
2 smart phones
1 Playstation 3
For normal circumstances (just the two of us at home) this works plenty. I tend to get the 50Mbps that I pay TWC for (at least, so speedtest.net tells me), I don't have significant packet loss or ping/latency spikes in any of the games I play. In the past, we've had some issues with the rokus and streaming services - especially netflix. This seems to have gotten better as of late, and when tested, both have worked better when connected via ethernet. Unfortunately, they're operating at more than 100ft away, and through two walls (I imagine that causes trouble with the wifi).
The issue is that when guests or family are over - which makes up for about three out of seven days - things get tricky. I am unsure of whether the router is just not suitable, or improperly configured, or if something is not functioning, but when the number of people added is greater than three, network performance and internet access speeds seem to drop like a stone. My working theory is that it is the number of devices - the most common guests consist of no less than two middle aged adults and a teenager, which added together means three smart phones, a tablet pc and two laptop pcs, and usually an xbox360. On holidays that baloons into four middle aged adults and four teenagers, each with a smart phone and laptop, two also have tablets, and they usually bring at least a couple of consoles over. When this happens, streaming video is virtually impossible and many complaints are had.
Putting aside the special cases and expected trouble from teenagers sapping bandwidth (which if anyone knows a solution for, I am all ears), I am hoping to increase stability of streaming services, especially under load - I figure it impossible during the holiday situations (outside of a dedicated internet connection for that purpose), but I was hoping there may be some way to account for the lesser case.
The location of the router is known to be suboptimal - when cable internet was installed, most devices were on the second floor, and wireless wasn't a consideration. The room it is in places it more or less two walls away from anything that would attempt to connect wirelessly, for which I commend the E2500's continued operation and connectivity. My major considerations at this point have been purchasing my own cable router - save five dollars a month anyway, and I've heard of Ubee routers having issues - or perhaps buying a new router/trying dd-wrt on the E2500. I don't know if either solution would help. Additionally, I was thinking of disabling the wifi on the E2500 and purchasing a wifi access point and mounting it to the ceiling downstairs. I think I could accomplish it with a 25 ft ethernet cable and a drill, and it would put both rokus in the line of sight of the access point, most other wireless devices on the other side of a single wall. My concern being whether this would be a good idea with so many devices (please forgive me if this is pure ignorance on my part). Very rarely will we have more than two devices connected via ethernet, usually a PC and either the playstation 3 or one of the rokus. I have a brother-in-law who swears by cisco routers and recommends I upgrade, and a friend who suggested a Mikrotik router if I decide to replace mine, but to try DD-wrt in the meantime. Nothing fancy is being done on the network - I pretty much configured it once for the connected and let it sit, aside from configuring a port forwarding range for a short time. There is now network attached storage, no printers or files shared, no servers running. Some of this might come in use when the kids around, as they like sharing files and running minecraft and similar game servers. I thank you all for any input you can offer - both in solving the problem, or determing whether it is equipment related, configuration related, or person related. Thanks in advance.