Here's a photo of a Lararium at Pompey: http://pompeiisites.org/en/comunicati/a-sumptuous-lararium-has-re-emerged-in-a-room-which-is-still-being-excavated-in-regio-v-at-pompeii/ And here, scroll down to the double lararium: https://www.pompeiiinpictures.com/pompeiiinpictures/R7/7%2003%2013.htm This blog has a photo with the household gods extant: https://holylandphotos.wordpress.com/2018/07/12/household-gods-and-christian-converts/
It is just lararium in nominative too, because it is a neuter noun. It doesn't have to do anything with accusative. You can remember cactus, octopus etc, which save their masculine ending. And I cannot remember any Latin word that was really borrowed in its accusative form
Great point with a slight adjustment: Latin accusatives, both singular and plural, led directly to the forms found in Spanish. Italian is a little more complicated: Italian singulars derived from Latin accusatives, but the plurals, which don't end in -s, derive from nominatives. E.g., Spanish cabra (sg) & cabras (pl) but Italian capra (sg) & capre (pl). Have a lingot. Cf. Joseph Solodow, Latin Alive (Cambridge) 234-35. The distinction you make between occidental and eastern Romance languages is helpful. Solodow (235) also notes that loss of final -s in Italian plurals is also the case in other eastern Romance languages.