Okay, couple of things.

Why the connection in ORD if you're on a paid ticket? Avoiding the early 747, or something else? The SFO-LHR nonstops are much more pleasant.

The 5.55 am arrival out of ORD (958) NEVER goes into holding patterns. Very few flights are authorized into Heathrow before the 6.00am curfew lifts, and UA958 is one of these. Therefore no hold unless it's running late.

You get off the plane, follow signs for Flight Connections. Down to corridors, up an escalator, down a moving walkway, down two more corridors, down some stairs and you'll be at the T2 (and T1) bus. Try not to get on the T4 bus, that would be a very bad idea.

The T1/T2 bus will bring you to the Flight Connections Center (FCC).

Here you will first clear security, and then be presented with AF checkin desks. The security part is tricky...the line can be huge. I wouldn't be terribly concerned with your outbound (1.5-2 hrs is plenty), but for your return I'd play it safe (2-2.5 hours).

When you check-in for your UA flight you need to show your AF reservation (as someone said above), and your bags will be interlined. You don't clear immigration or customs in the UK, you do both in France. When you check-in for your AF flight in the FCC, YOU MUST TELL THEM that you have bags being interlined and give them your bag tag numbers. If you don't, there's a very high chance your bags won't make it. On the outbound, yes, UA's baggage policies will apply.

On the return, same but in reverse. Land T2, Bus to T3, Security, UA checkin. No immigration on customs (well, French exit immigration) until you reach the US. AF will gladly interline your bags with a UA receipt, but AF's very restrictive bag policies for intra-European flights will apply. This will prove to be VERY expensive for you. I think you're absolutely nuts, no offense, not to have just booked a revenue ticket on UA SFO-LHRx-CDG-ORD-SFO. You would've saved yourself a lot of hassle, and I also think you wasted 20-25k points on an LHR-CDG roundtrip which can be found for $150 very easily with any sort of advance notice. You might consider still doing this, and just buying a one way LHR-CDG and swallowing the change fees. Are you staying in London at all? If not, why aren't you just on an SFO-CDG revenue round trip and saving yourself a TON of hassle and 50 some odd k miles?