Hi folks, I've been to Mercury Ballroom to view the wedding set-up twice, one with 27 tables, one with 29 tables.
Personally, the 29 tables-set up was very cramped, thus I guess if you are talking abt 30 tables, it will be worse, very cramped up near the stage. So for 29 tables and above, Venus Ballroom will be a much better choice.
I have around 30 tables, so I took Venus, was also advised by my coordinator to do so, as the numbers might just go up as AD draws nearer.
Shining Hoshi, I have the same problem as you. My FIL increased the number of tables, initially we were just looking at Jupiter as we only estimated around 22 tables, then we "upgraded" to Mercury 2 weeks later when he added further and said 27 tables. Then he said he might need to invite some more ppl, so we ended up taking up Venus which can accomodate 30 tables and above. So you might wanna consider taking Venus instead if u can't compromise with your in-laws on this issue yet.