Very compositional, but then so are (the far more common) 一時間以内  and 一年以内. I guess this is harmless.

@@ -5,1 +5,1 @@
-<keb>数秒以内に</keb>
+<keb>数秒以内</keb>
@@ -8,1 +8,1 @@
-<reb>すうびょういないに</reb>
+<reb>すうびょういない</reb>
@@ -12,0 +12,1 @@
+<pos>&adv;</pos>

See below.

Very common usage. More than 500k Google hits. Your [A] link confirms with some examples. But still only "a + b". However, it is good to see more "a + b" entries because they help the non-expert get a feel for usage; especially when the entry represents a complete expression usually used at the beginning of a statement, much the same as in English. You also have similar entries on site [one of which encouraged me to keep the "に"　in the submission], for example:-

一時間以内 【いちじかんいない】 (n) within one hour
一年以内に 【いちねんいないに】 (adv) within a year