Postremo, ut iam agendae rei tempus visum est, cinctus agmine armatorum in forum irrupit. Inde, omnibus perculsis pavore, in regia sede pro Curia sedens patres in Curiam per praeconem ‘ad regem Tarquinium’ cogi iussit.
I spent some time trying to figure out what or who the participle ‘sedens’ relates to.
Does this mean 'Then, everyone having been struck with terror, it [the armed force] ordered, via the praeconem [speaker] [who was - (is there an implied ‘qui’? to make sedens nominative?)] sitting on the royal throne, that the senators were instructed [cogi] to side with King Tarquin [ad regem Tarquinium] ?
…the senators give in (the text in square brackets is Orberg’s guide):
Convenere extemplo, alii iam ante ad hoc praeparati [prae-parare], alii metu coacti et iam de Servio actum rati [de Servio actum esse: rem Servii actam esse].
‘…iam de Servio actum rati’
believing that matters (it’s so hard to translate res!) had moved away from Servius…?
Assuming my sense is right it’s amazing how compact this phrasing is..
so he ordered the senatores by means of the Speaker (per praeconem) - i.e. he told the speaker to announce to the senators what they must do..?
Yes. Praeco = herald.
‘…iam de Servio actum rati’
This is impersonal. de here means “concerning”. Something like “thinking that official action had already been taken concerning Servius”, i.e., that he had already been officially removed from the kingship.