Seller put in new subfloor himself...should I be concerned?

[Nicholas Morgan]

Hello Everyone. 

I apologize if this isn't the best/proper forum for this question. 

I currently have a Triplex under contract (technically it's a duplex with a single family home right next door). 

I had a house inspection and had a list of items to be fixed/assessed and the seller has agreed to take care of all of them (list will be at end of post) except for paying for a structural engineer to come assess the structures. 

The duplex has a noticeably slanted floor. It's not extremely awful, but it is noticeable that it is not level. And on the second floor there's a very noticeable bump in the flooring where I assume a beam to run. (due to the ceiling/flooring, it isn't possible to get access to seeing this beam). It is reasons such as this that made us want a structural engineer to assess the property. The inspector speculates that house has settled over the 60-70 years the house has been standing. We speculate the ceiling has been replaced due to no cracking. The walls have painted paneling so there's no cracked drywall. However, there is one closet door that I recall not closing properly. Perhaps due to shifting? 

I find out today that when the seller purchased the property he replaced the subfloor of the duplex. This was not disclosed prior. Should I be concerned? My agent thinks he just doesn't want to pay for a structural engineer to come out. However, I feel that maybe he's afraid of what they might find. He did this work himself. I'm not sure of the seller's background. Perhaps he's been in construction for many years and everything is fine. But perhaps he's just seen some YouTube videos and doesn't have much experience. I know he's selling the property to help fund his current project of building a house (perhaps he's building it himself).

I need to prepare a list of questions to email the seller by the end of the day. I'm just not 100% sure what to ask. Here's what I've got so far. What questions should I add to the list? 

- When (year/month) was the subfloor put in?

- Why was it deemed necessary to replace? 

- What is the seller's profession/experience with construction?

- Why was the floor/structure not properly leveled at this time? 

Thanks in advance! Not sure what to make of this. 

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I can attach the inspection report, but here is a brief summary of what we asked the seller to fix: 

- Electrical:
---- Secure wiring properly that goes to AC compressor outside
---- replace some GFCI
---- Add smoke detectors to all units
---- Fix breaker box because there are missing breakers where live AC is exposed
- Plumbing:
---- Hot water knob on bathtub didn't work. Hot water in apt did, but not that knob
---- Missing knob for under-sink water valve
- Venting
---- Install proper dryer vent port in wall
- HVAC
---- One furnace did not kick on. Have it assessed and fixed/replaced
- Trim Tree Limbs
---- Some tree limbs are touching the roof
---- Some tree limbs are hanging and seem dangerous
- Paint eves of house where paint is peeling
- Clean gutters
- Fix poorly attached handrail
- Fix/replace windows to be functional and not have a hole through it
- Have structural engineer come out an assess the structures
---- The floors are unlevel in the duplex and is slightly concerning.
---- House inspector believes house has settled over the last 60 years, but would like to see a professional give an assessment 

Thanks,

Nicholas