Jane being in Bingley's house would be of little use for Mrs Bennet's purposes while she is not in his company. Her plan, IMO, is that Jane will arrive before the rain starts but will have to stay the night. Then she will be there when the gentlemen return from dining with the officers and for breakfast the next morning and, if the rain persists, will probably come home in Bingley's carriage in the course of the day. Being in an optimistic mode at the time she makes her plan, she might even hope Bingley will offer to escort Jane home.