I've seen some videos on youtube where someone recommend a tar like substance to fill those holes and prevent moisture getting in.

We don’t know any PMCs to recommend in the area mentioned, but since selecting the wrong PMC is usually more harmful than selecting a bad tenant, you might want to read our series about “How to Screen a PMC Better than a Tenant”:https://www.biggerpockets.com/member-blogs/3094/91877-how-to-screen-a-pmc-better-than-a-tenant-part-1-services-and-processesWe recommend you get management contracts from several PMCs and compare the services they cover and, more importantly, what they each DO NOT cover.

But if your doing a few a year, I would applaud you for doing things yourself.Another thing I would advise, if you are going to take on a task that could cause harm, such as electrical, or HVAC, structural, etc.... make sure you take the time to educate yourself and make sure you do the task correctly.

Don't do partial payments to a bunch (I've seen old 'quiet accounts' wake back up and start reporting late/partial payments when folks do that, doing more harm than good), pick creditors and pay them off entirely, smallest and most recent first. 

Image over substance. 

Would it be of any advantage to the harmed investor?

We don’t know any PMCs to recommend in the area mentioned, but since selecting the wrong PMC is usually more harmful than selecting a bad tenant, you might want to read our series about “How to Screen a PMC Better than a Tenant”:https://www.biggerpockets.com/member-blogs/3094/91877-how-to-screen-a-pmc-better-than-a-tenant-part-1-services-and-processesEven if someone give you a referral, don’t make the mistake of assuming the PMC will meet your expectations, just because they met the expectations of the referral source.We recommend you get management contracts from several PMCs and compare the services they cover and, more importantly, what they each DO NOT cover.

No substance.

If it has been listed on the market for 2+ months, then I don´t see the harm in making an offer under the listing price.You´d have to wonder why it is listed so far below market value in the first place though.

Sub2 deals aren't that complicated, but you do need to understand what you're doing, you can do a lot of harm to the sellers doing it wrong and some states have some laws in place because people have gotten burned.