Ok, First you changed a tire but have a stripped bolt and or nut. Both can be gotten from a auto parts store, you can just knock the old bolt out, they will tell you how. No special tools.

Next I believe you are saying you have a vibration comming from rear wheel at speed. If its at speed and durring a turn I would look for tire balance, Bad wheel bearing, loose lug nuts. pretty easy if i understood you correctly.

Oil leak, it might not be engine oil, you have transmission and power steering, or even antifreeze. In any case have engine steam cleaned (Cheap) drive for a hr, pull up on curb to get you extra room, put in park with break on, carefully crawl under with flash light and go see, then tell us.

I dont think its related to oil leak. But look on internet or call dealership as most cars have a way you can read the code for free, on mine i turn ignition switch off/on 4 times under a minute and dash lites flash codes. Then youll know.

Yes, steam clean it. Look in local phone book, or buy engine cleaner in spray can from auto store and goto car wash (do it yourself), Let engine cool down, spray clean it followed by washing off with water spray.

Get under car and see if where struts connects to frame the bushings are wipped/worn out. that would be the noise, push up and down on back of car, is it cushioned ? like dampened, thats the struts, If their shot you’ll get bouncy bouncy instead of the dampening type push.

feel free to email, but try a little harder to explain in the most detial you can