Double bar lines are in fact system items and they will show up in the
system items dialog. Just press Alt+F2, select the double bar line in the
list and press Enter. You'll be put back in the score and the double bar
line will be selected.

To turn a double bar line back into a single bar line you only need to
delete the double bar line. The original single bar line will be put back
automatically.

It's worth taking a minute to look at this more closely. The double bar line
is in fact a "special" bar line which is defined by the user, instead of the
normal "single" bar lines which Sibelius puts in automatically. If you
remove a special bar line then the normal bar line is put back. You can hide
the automatic bar lines completely if you want but this is only necessary
for passages in "free time" where there is no time signature.

Bar numbers work in a similar way. Sibelius draws these automatically and
there options in the engraving rules dialog which control which bars have
numbers drawn for them. The default is to draw a bar number above the first
bar line on each system but you can change this to put a number above every
5th bar (or any other ordinal you choose).

Sighted users can select the automatic bar lines and bar numbers and move
them around with the mouse, but we can't select them by tabbing or through
any plug-in. It is possible to select these items by using the advanced
filters but the plug-in can't get any information about these items so
you'll be essentially working in the dark.

You can create a bar number change (which is a little like a special bar
line). When you create a change, the bars which follow it will be renumbered
starting from the change. You can select these bar number changes (which are
system items) by using the system items dialog or through the advanced
filters.

In fact, if you have a piece with a pick-up bar, you will always see a bar
number change in the first bar of the piece. The change in the first bar
sets the bar number to 0 instead of 1, which is the default.