This next sentence reads
Claire (Caitriona Balfe), Jamie Fraser (Sam Heughan) and Roger (Richard Rankin) turned up to stop Bonnet getting away
I think it should be "From getting away" I feel it's incomplete without it.
The sentence is correct as written. To see how the syntax works, you could assume that a preposition has been omitted, as you did:
Claire (Caitriona Balfe), Jamie Fraser (Sam Heughan) and Roger (Richard Rankin) turned up to stop Bonnet from getting away.
Another option is:
Claire (Caitriona Balfe), Jamie Fraser (Sam Heughan) and Roger (Richard Rankin) turned up to stop Bonnet's getting away.
A possessive noun modifying a gerund is often changed to the noun's base form. (Doing so often sounds more natural to fluent English speakers than keeping the possessive form, as in this case.) If you're interested in that topic, it's been discussed many times on this website. (I recommend searching for the tags "gerunds" and "possessives". For example, here is one ELL question that deals with this issue: Possessive form of the noun...why is it incorrect?)