You sure it's just beta? It looks finished stuff for me! But I'll answer your questions:
1. Only found 2 locations that were "floating" and couldn't be reached by car, and 1 that was "stuck" on the wall (inside building 269). I don't remember their names but I made the map of them:
Otherwise, no actual bugs. It's working perfectly.
2. I like it only because it's a the classic Taxi Mission back to GTA4. Period.
3. Allow the Cavalcade to be a taxi too, as [spoiler]Roman's new taxis are Cavalcades when they finally go to Algoquin[/spoiler].
4. The tip bar is in ok speed, if you don't mess during the way you can still have a fat tip. There's only one problem with it, maybe not mod's problem but a game limitation: let's suppose you're in south Alderney and the destination is in south Algoquin - places are close, but physically imposible to get there quickly with a car (you have to do the long way thru Booth Tunnel). Maybe if there's a way to calculate the tip based on the GPS route, it would be neat. But if there's isn't, don't worry because this situation just happened 1% of the times I played it.
5. Nah. Nitro was cool on GTASA, but it doesn't fit on modern Liberty City. An all-proof black vehicle, parked near to each safehouse (not on the parking lot ) would be cool tho.
6. It's ok, but it could also show the total number of customers, and not only customers in a row.
7. No flickering.
8. I'm using with other mods and I found no problem with them.