As of 2019 the answer to #1 and #2 was "Yes", and I've found references to the 2-for-1 deals online even more recently, so I believe not much has changed. It's possible that--at least for some sights--it is now possible, due to COVID, to get entry time slots--or perhaps actual tickets--online in advance under the 2-for-1 deal. I'm generally a solo traveler and haven't checked on that.
I do recall seeing some time limitations on use of the deal for a few sights--places where the offer wasn't good on weekends or wasn't good during peak summer months. Advise your friends to read all the online 2-for-1 info on the places for which they are considering taking advantage of the 2-for-1 deal.
Unless there is a way to take care of the ticketing ahead of time, it is highly questionable whether it makes sense at some highly-popular sights to use the offer if you're the typical time-pressed traveler; it means standing in what could be quite a long ticket line. One such place is the Churchill War Rooms, which also has the potential to sell out. On the other hand, it was a piece of cake to buy 2-for-1 Kew Gardens tickets on site.
One other thing I should mention is that in the last few years I've visited London both with an Oyster Card and with a paper weekly pass. The Oyster Card worked like a charm. The paper pass didn't always open the fare gates as it was supposed to. That was only a minor irritant; fortunately, there was always an employee at hand when I needed one. However, if I somehow knew 100% for sure that I would only be making use of one 2-for-1 deal, I might not bother to get the weekly travel card, which is likely to take a bit longer than just buying and loading an Oyster Card. The modest savings from a single sight might not be worth the extra time. I don't know that it is still true, but I remember seeing a few places back in 2019 that gave about a 10% discount for buying online tickets. In those cases, the 2-for-1 deal effectively saves you 80% of the cost of the ticket you don't need to buy rather than the full 100%.