The US never adopted teletext on any significant scale but they had the V chip activated using data on line 21 of the VBI.
No - though a number of US stations ran services in the early 80s. Most used the 525/60 version of WST (i.e. the same system used in the UK modified for NTSC) - and I think TBS in particular ran a service. Zenith made TVs that had decoders, but when they re-engineered to add stereo functionality to their range, the teletext functionality was dropped, as by then it was clear it was going nowhere.
Some other stations in North America used a different system - NABTS (North American Broadcast Text Service) - which I think was originated in Canada and may have been linked to the French Antiope service a bit (France had their own bespoke teletext standard for a number of years before switching to WST).
Subtitles (aka Closed Captions) in North America are delivered using a separate system (much cruder, but also more robust, so it survived VHS recording) and the V-Chip system was integrated into this standard AIUI.
There have been instances of where teletext has been used for bulk data transfer between computers by enabling data to be inserted into a communications channel designed for video. In some cases it has used video links inaccessible to the public (often with no video, just sync pulses) and other times on a broadcast TV network which would result in random data appearing on a TV screen in teletext mode.
Yes - you could use as many lines as you liked with some WST decoders, allowing a much faster page access and bigger carousels. ISTR Reuters had a ring-main system that included full-screen teletext that allowed for very fast access to a lot of dealing room/markets data.
Other data-over-video standards have been used over the years. The Sony F1 system carried digital stereo audio over a vision circuit, and allowed digital audio to be recorded to VHS or Betamax. There was a similar system used for UMatic 3/4" industrial tape that was a key part of CD mastering for many years.
There were also systems that let you use VHS tapes for PC file backup in the days before CD-R/DVD-R recorders.
There were also other data formats used alongside CEEFAX in the UK. The BBC sold off some of its VBI space to allow for Datacast, which was used to broadcast price lists to shops like Dixons (in the days before broadband connectivity), and ISTR that PresFax (in a number of incarnations) sent data in blanking (though I think the final iteration of PresFax actually sent packets in the general CEEFAX stream that only PresFax decoders bothered with) (PresFax carried junction data to the English regions, with the original version also carrying junction scripts AIUI)