Not surprised about the bolts. In another forum I saw comments about the basic single speed Hoptown/Tilt and it was very low end. The bottom bracket didn't fit the bottom bracket shell it was loose and the single speed freewheel was a very low grade indian made freewheel normally found on small children's bikes. Very poor quality and constantly clicking. Considering the bike I think was £130 here in the UK and had no fitted accessories and such a simple drivetrain with no derailleur, shifter etc I think they could have afforded to have used better components. The trouble is many people buy and recommend by brand but the reality is most brands have good and bad bikes and that bike was I feel unacceptable quality for the price. Saying that £130 isn't a lot of money for a bike but you do get competing bikes at the same price point with better components all round.
If you did buy the Tilt 500 with 7 speed gears and hate gripshifters like me you can get a simple tourney 7 speed shifter set for low money from ebay.
Mountain Bike SL-TX30 Thumb Gear Shifter 3x7 Speed Shift Lever Set for Shimano | eBay
Unfortunately they come in pairs with the front shifter as well which you won't need but its cheap money. I have big hands and a strong grip and really don't get on with gripshifters and often find I end up changing gear when I don't want to which can be dangerous.
If you don't change the rear wheel and keep with stock a hardened chromoly rear axle would be a good upgrade. The type often used on bmx bikes. Just means you can eliminate or improve many of the issues with freewheel hubs. By preventing the rear axle getting a slight bend by using a stronger axle you can improve shifting, braking and reduce bearings wear plus have the ability to do pavement drops etc. If you have soft axle nuts you probably have a soft weak axle as well that they are attached to and such low grade components are a liability. A decent rear axle isn't hugely expensive but can make huge improvements to the bike in how it functions and its long term reliability. You don't normally have to upgrade the front axle as there is far less weight on it.