euroline point to point tickets

Our experience with the bus system in Spain is exactly as you asked.

Yes, for the most part. Bus travel is very similar to Greyhound in the US - terminals, ticket windows, etc. However, the rules of travel still apply. You might find some bus tix to be impacted & on short supply depending on cicumstances: day of the week, freq. of route, popularity.
I'm not sure what you mean by 'public transit convenience'. Local or regional destinations are more frequent & will allow you to be more flexible. But if you are going from Barcelona to Prague there will be 1 or 2 buses per week, so you will need to do some amount of planning.
I'm proof you can do Europe by bus - I used a 30 day Eurolines pass on my first solo trip. Msg me if you'd like to ask other questions.

you can show up at the station but i wouldnt there may be no room so you have to pick up your ticket in advance. but if you like a gamble walk up. buses are so unreliable take the train.

So If we grab a shuttle from Charles de Gaulle airport in Paris to the bus station depot. I believe its called "Routière in Bagnolet (métro Gallieni)"

We should in theory be able to walk up to a ticket window, find out the time for the next bus departure and pay cash Euros for our tickets. ( All this in our best mangled french) Unless we find an english speaking ticket agent.

Incidentally, has anybody ever been to the Paris bus depot? Is Eurolines the only bus ticket one can buy, or is there a number of different companies offering competing services?

According to the Google search I just did, the name of the station is Gare Routiere Internationale de Paris-Gallieni.

Have you gone to www.eurolines.com and checked the timetables? You may find that buses don't run as frequently as you would like.

To get to Gallieni from CDG, take the RER B to Gare du Nord. Transfer to Metro line 5 (direction Place d'Italie) and take it to Republique. Transfer to Metro line 3 (direction Gallieni) and take it to Gallieni. The fare is €8.20.

If you don't want to be bothered with transfers, you can take Bus #351 directly from CDG Terminal 3 to Gallieni. I think the fare is three single tickets (€4.50).

We used ALSA buses in Spain and Eurolines virtually evrywhere else, when we didnt take trains. Sometimes buses are faster, sometimes they are alternatives to sold-out trains. Invariably, they are cheaper.
Yes, you can walk up on day of travel and get tix. Be aware that popular routes can be sold out, just as with trains.We have never experienced hassles in using buses.