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According to MileageMonkey it is doable as an xONE3, with a few transits through MIA to get from YYZ to the Carribean:

amm-lhr-hkg-cmb-hkg-ktm-hkg-yyz-mia-bze-mia-yyz-mia-uvf-lgw-tun-amm

Note the above uses all 16 segments allowed

The rules on stopovers are easy - you can stopover anywhere on the itinerary EXCEPT that you are limited to only two stopovers* in your continent of origin. (* - further limited to a maximum of one outbound and one inbound in the country of origin)
So on the above itinerary, you can stopover three times in HKG and also three times in MIA

Your continent of origin is Europe, which for the purposes of an xONEx ticket includes the Middle East and North Africa. So you can only have two stopovers between your three 'European' ports: LHR, LGW and TUN. Presumably you'd make one of the London stops into a transit (i.e. <24hrs); this will also save you one set of the exorbitant UK departure taxes.

FYI the ticket rules are here