Advantages of inviting them:
--Easier than confronting MIL, or them
--They can fit into your venue and budget
--Possible rekindling of relationships with them
--No worries about being rude
Disadvantages of inviting them:
--MIL thinks she has power over your wedding, and possibly other parts of your life
--Possibility of MIL continuing to invite other unauthorized people, who may not fit into your venue and budget
--More anxiety for you and LordL on your wedding day
Hope I've got those right... Don't look at the total number of points, but rather the magnitude/likelihood of each point. If your MIL is the type who invited people just because she got overly enthusiastic, and you can tell her to not invite anyone else and trust she won't, and the extra guests can be easily accommodated, maybe issuing them official invitations would be the best route for you.
On the other hand, if your MIL is more the controlling, dictatorial type, who may have done this deliberately to exercise power over you, and even the thought of these extra people being official guests makes you anxious, then I would say don't invite them. As PP said, you're not uninviting them; they were never really invited in the first place, and the only one who would be rude is MIL (at best, clueless).
Top solution, if not inviting them, is to have MIL clean up her mess by contacting each of the people and clearly stating they are not invited, hopefully with the appropriate non-inflammatory wording. As extra protection, especially considering they know enough details to just show up, and are already making plans, I think LordL should also contact them, as a follow-up, with appropriate wording. Not because it's his obligation (it's his mother's), but to make extra sure MIL passes the information and sentiment on, and to be a gracious non-host to his non-guests, in a sense.