Hi,
About the proxy objects, these require their appropriate object enablers to be viewed properly in the target application. As far as the error message implies, you need Autodesk Civil 3D Object Enabler app.
You can consult this page to download the latest version:
https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Download-Object-En...
This requires you to login with your Autodesk ID and since you don't have an active subscription, I presume the downloader will not present you with any software, but take your chance. Below is a link to the download page of Oe 2022.
https://www.autodesk.com/support/technical/article/caas/tsarticles/ts/2L799jWxGbokAksZurR08P.html
About the corrupt objects, I don't think it has anything to do with the missing proxy objects error. It resembles a roundoff upon unit conversion issue.
3dsmax's level of precision is not unlimited. When you bring a very large object into a scene with tiny extents or a tiny object into a vast extents of space or change the system unit scale so that a disruptive rounding takes place, you might have artifacts. Both visual and topologically.
I believe your scene has such kind of scaling disruption. As if the Autocad scene is built in meters and brought into a mm scene with the badly matching import setting.
Check for those. Check 3dsmax's system units. Also Autocad's. You may consider creating simple primitives and import them in the first place to see if any of the scale values are problematic.
