Firstly, IANAL but I had a nightmarishly shady landlord a few years ago who forced me to become as close an expert in the "landlord-tenant rights" department as a layperson can be. I practically had the my state's (TX) landlord-tenant statutes memorized and received legal assistance to ensure my understanding of those statutes. Also, I'm kind of fascinated by law and legal matters so I read a lot about the subject (which probably helped me in my aforementioned landlord issues since I rather enjoyed reading through the statutes and doing the research). I know MD's tenant laws are different but since you haven't received many answers so far, I thought I'd chime in.
The exact wording of the relevant statute in the Code of Maryland is as follows:
(f) (1) (i) The security deposit, or any portion thereof, may be withheld for unpaid rent, damage due to breach of lease or for damage by the tenant or the tenant’s family, agents, employees, guests or invitees in excess of ordinary wear and tear to the leased premises, common areas, major appliances, and furnishings owned by the landlord
The burden of proof lies with the party making claims against and seeking damages from you. Although it would help avoid this entire potential mess in the first place, you are not responsible for retaining and providing them with the proof they need to substantiate their claims against you. Your landlord cannot charge you for existing damages not caused by you (and on a related note, since you expressed concern about them doing so, they also cannot and should not charge you for normal/expected wear & tear). If they charge you for any existing damages, they'll have to be able to prove in court that the damages they're claiming you owe were caused by you . Since they lost/do not have the move-in inspection checklist as they indicated in the voicemail, they're going to have one hell of a time proving their claims against you in court. Definitely keep that voicemail.
It sounds like the former manager was a amiable/helpful person - you maybe get in touch with them somehow (assuming they did the move-in inspection) and see if they still happen to have a copy or if they remember/can vouch for anything? Other than that, I'm not sure what more you can do to prevent them from unjustly charging you for all damages but it sounds like they would have one hell of a time proving their case against you in court, so I'd probably take it to that level if the bill is substantial and your situation allows you to do so.