I dont think you can hang your homes problems on a 2 year old inspection A home inspection can only point out things that are wrong at the time of inspection. Kind of like buying a used car holding the seller responsible for a fan belt break 2 years after the sale.
The septic system is either operational at the time of inspection or not.... The drainage problem most probably has nothing to do with the home but rather the septic system. The septic system is not part of the home or put in place by the home owner. It is installed by a local septic contactor independent of the home. You may just need to have the tank pumped... which is something you should do periodically. The frequency would depend on the soil types and number of feet of drain field. Call a local septic contractor and ask how often it is necessary to pump tanks in your area.
See the answer posted here
http://mfdhousing.com/phorum5/read.php? ... #msg-79062 for greater details about septic system repair and maintenance
The problem at the back door is almost certainly from moisture. Most older homes were built with particle board floors. This type of decking will start to disintegrate the fist time it gets wet. If the floor was showing no visible sign of problems at the time of inspection more likely there has been moisture introduced in the recent past.
The floor problem is not difficult to repair. Take a look at Floor Repair Info Kit from Aberdeen Repair This kit includes a 25 minute VHS video about repairing the floor in your bathroom underneath your toilet, the hardest spot in most homes to make repairs... and a booklet on floor repair. When moisture has been absorbed by the floor the good edge will continue to draw moisture from the damaged section so Dont let this problem continue as it will grow.
As far as cracks in the dry wall... all structures continue to settle which is what generally causes cracks in drywall. Cracked drywall alone is not a structural unsoundness. Drywall cracks require little skill to repair and only a few simple tools and supplies. For detailed instructions see How to fix a crack in drywall
The type of problems your talking about are not uncommon in any structure, factory built, site built, residential or commercial. When you buy a new home you expect that all the parts are brand new and dont expect to have much in the way of maintenance or serious repairs for years, till the houses components start to age. If you do find problems in a new home they are most often covered by the builders warranty. When you purchase a used home you get a better price because you are purchasing something that will need more repair and maintenance that a new one would. .