Coaches often do have a 3 point belt these days. I'd be happy to use a high back booster or even a backless booster on a coach personally. Because coaches are that much bigger and heavier than most other vehicles, they tend to absorb less of the impact in a crash so there is less need to be rear facing. Rollovers can be a concern and a closely fitting seatbelt is helpful in a rollover situation, but this is also incredibly rare.
Maxi Cosi have a folding HBB called the Tanza. I agree a loose one can be prone to breakage - we had this happen once with a Britax HBB (so nothing flimsy!) but if it's folded up and in a travel bag, it should be more robust to any impact or may fit into cabin luggage size guidelines.
The Trunki also has a plastic shoulder belt guide which can help with comfort. I would not add anything which pulls the shoulder belt down by pulling the lap part of the belt up - these are really bad for accident performance. You want the lap belt to be snug on the hips or top of thighs as far as possible.
Retractable lap belts are arguably completely pointless for anything at all - if using a lap belt it should ideally be static and adjustable (like a plane belt or the old style lap belts you used to get in the middle seat in cars). I've never seen this in a coach, though. TBH if a retractable lap belt is the only restraint, I would still use it.
When backless boosters were first invented in the 70s and 80s, they did originally say they could be used with lap belts only. Some research/crash tests in the early 90s found that this was not advisable because with the lack of upper body restraint, a child positioned higher in the seat tends to move further forwards, which is why backless boosters since that point have advised never to use with a lap belt, only a 3 point belt. There was another study done in 2009 which looked at real world accidents and found the opposite effect IRL - that boosters with a lap belt very slightly improve outcomes over a lap belt alone, and they put this down to the fact that the small reduction in head excursion due to sitting on the seat itself isn't enough to prevent impact on other items in the car. However, it wasn't significant enough to change advice mainly due to the small numbers in the sample (only 2% of the children over a 10 year period of accident data had this restraint combination). So it's basically unknown whether a backless booster with lap belt is better or worse than just a lap belt on its own.
If you end up with a child on your lap at any point, remember a seatbelt must only be used for one person, so the heaviest person should wear it and carry the child on their lap if there is no separate seatbelt for the child.
Sorry if this sounds doom and gloom, I tried to stick to facts but that can come across a bit focusing on the worst case scenario. Overall, I totally agree with the fact it's a tiny proportion of the time they spend travelling in any vehicle and it doesn't help to be overly risk averse. Sometimes you just have to do what you can to reduce a risk, but let go of perfection and basically say you know what, it's very unlikely anything will happen - let's focus on the holiday.