I can sympathise with you to an extent - any small club which is not strongly established will have its probelms; and any characters who behave inapprpriately will have a larger influence over a smaller club.
Have you thought of setting up a proper organisational system? Have the instructor as the chairman to start off with. Have monthly meetings, with a proper itinerary, and discuss problems. You need to make sure;
i. there is someone in authority who is responsible for everyone else
ii. there is someone who deals with the administration (mats, money, advertising)
ii. that you encourage new people into the club
If you don't sort this out it's likely training will just degenerate. In terms of authority, you should have insurance, and in the UK the instructor has to have their own insurance - ultimately the instructor is responsible for any injuries or deaths on the mat. If the instructor is not always there you need to get additional insurance for another instructor (who is then responsible).
All authority defers to them, since they will be the ones in prison for negligence. Don't forget; aikido techniques can and do kill people. Hope this helps,