I can't grind my teeth due to overbite. can my tight lower bite splint have caused numerous crazelines on my lower incisors, or do i just have weak tooth enamel?

Craze lines: It's unlikely that you bitesplint is tight enough to cause any damage to your teeth, unless that is, you don't wear them every night. Even people with overbites can grind their teeth, especially when sleeping. The craze lines where probably already there from previous bruxism habit. The best one to answer your questions is your own dentist who can personally examine you & knows your dental history.

Probably not: Craze lines running vertically through the enamel are usually seen where there is edge to edge contact between maxillary and mandibular teeth, and are more pronounced in bruxers. Since we cannot examine your occlusion over the internet, have your dentist show you how this happened in your mouth.

Have guard eval: It is always good to have your dentist periodically evaluate your bitesplint and/or guard. You may need adjustments, and generally, know that your guard is meant to take the brunt of your bruxism/grinding/and/or clenching, so as to help protect your teeth. If it's too tight, get it checked!

Not really: Many people have 'craze lines' on there teeth. I don't think your bite splint will cause that!