I agree with your reading.
Lists of witness are usually in the dative: Hijs testibus Willelmo Johanni Thome Marie et Agneti
If you want to add "with others" I would think others will be ablative - Hijs testibus Willelmo Johanni Thome Marie et Agneti alijs
(Alijs is both dative and ablative, which isn't helpful here.)
These are sureties but if this analysis is correct the case of the others shouldn't be affected.
It's fairly common to find contraction marks for which there's no apparent need. This looks to be one. At best this is pretty half-baked Latin.