"- do I need to clear customs and go get my BMI boarding pass at the land side of the terminal ?"
No. You can get your boarding pass at the lounge or the gate, or one of the information desks in Terminal 1 (BD's terminal). Follow the black "Flight Connections" signs from your arrival gate. This will take you on a (very) long walk which will end at a security checkpoint. You may be asked to show your ticket (or eticket receipt) here. If you find yourself in the immigration hall you are going the wrong way!
You will then be able to get a bus from Terminal 3(UA) to the Flight Connections Centre (a building joined onto Terminals 1 and 2). From here follow the signs for Terminal 1 International Departures, which should take you along a moving walkway and down some stairs into Terminal 1.
Go to the lounge if time allows (which it probably will since the CDG flights (all airlines) have an apalling punctuality record), or else check the screens for your gate.
- if not, what do I do with my carry-on luggage: 22" rollaboard and a good size laptop case ? My understanding is that BMI allows only one bag in Y. What do I have to do with my other one? Check it at the gate? Is it possible?
Your understanding is correct though if the flight is not too full they may let you on with both. You can check luggage at the gate. It goes down a chute and straight onto the plane.
- would I be allowed to visit BMI lounge as *G ? How far is this lounge from the UA Red Carpet Club? Which one is better ?
Absolutely. There are 2 lounges in Terminal 1 International, near gate 14. The Business Lounge is for business class passengers, elite BD frequent flyers and Star Gold. Opposite that on the left hand side is the Diamond Club Lounge, for elite BD frequent flyers and Star Gold only. The latter tends to be less crowded though both offer similar facilities and generally very nice staff. I understand that the Diamond Club lounge has shower facilities though I have to admit I've never used them or seen them.
Both UA and BD lounges are similar though I prefer BD at LHR (no internet access but otherwise good). You don't actually have a choice because UA will be prohibited from admitting you to the RCC unless you have an onward flight from Terminal 3 (which you don't).
- would I get any break with my sitting assignment as *G ? Or is it "free for all" at the gate?
The staff will do their best to accommodate your request i.e. you will get a break where possible. You may find the seat next to you blocked off but that depends on how full the flight is. If the flight is packed and you're the last to check in then you may still end up in a middle seat at the back.
Note that the CDG flight is short (less than an hour) so you will only get free drinks (alcoholic and non-alcoholic) and a biscuit/peanuts. Similar to domestic UA.
Ground staff at CDG are significantly less friendly than their LHR colleagues in my experience.
Thanks in advance for you help.
No problem!