While it's not directly possible in the current version with its own keystroke, there is another very simple way to accomplish this.

First, be located in the row on which you want to insert a row above or below.

Then do the following (keys are on Mac, so translate as necessary for your OS):

1. Ctrl + Option + I - invokes the Insert menu
2. Then hit the R key for Row Above, or the B key for Row Below

I guess this way may actually be one less keystroke than how it was done before, so that's a win right there!

Alternatively, you can also just use your cursor keys (to me this is easier, even if it's more keystrokes):

• ↓ (down arrow once) for Row Above
• ↓↓ (down arrow twice) for Row Below
• Hit ⏎ (Enter).

Hope this helps!

In an arrangement where each of N user's changes propagate to all the other users' screens, that incurs N² propagation costs (network messages, recalculations, and display updates). That grows rapidly as N grows. So if N > 3 you want to avoid it for sure.

A separate sheet per user might avoid most of the updates but the best bet is to use a separate spreadsheet document for each user.

Given that, using a separate Google Account per user might improve performance a bit by avoiding synchronized updates to some shared state (e.g. the user's document list). Also this is the normal case, unlike one account editing docs from many tablets at once, and any optimizations will favor the normal case. You'd have to measure it to be sure, but my semi-informed bet is that you won't notice the performance difference.

Giving each user a separate login account would help with tracking changes, but that might not be worth your setup work.

Idea: If all the columns are uniform within each sheet, that is, if the data is like a simple sequence of records, look into using Google Fusion Tables in place of spreadsheets. Fusion Tables scale up to very large data sets since the rows are independent of each other. You can "publish" columns from one table data to other tables.

Idea: If this is not a temporary application, consider replacing the spreadsheets with a custom implementation as a web app or native Android apps. Even then, it's good to prototype your application with spreadsheets as a way to discover what really matters to your use.