Ask for proof of everything done and check local laws work should not be carried out without owners permission. Extra stuff should be explained to you before even ordering the parts.

If the cobalt isn’t ideal sell that. I personally love owning multiple vehicles. If the accord is reliable ( which I bet it is ) I vote keeping it. All depends on financial circumstances.

Temperature getting colder usually massively increases my mpg but can do the opposite affect. As you said tyre pressures being off can affect mpg by a decent amount. The type of fuel put in can hugely impact it. I found in my area my vehicle hates Tesco fuel but loves Morrisons and BP fuel ( difference in mpg is 10 ). Also service history. If the vehicle is neglected that won’t help either. Spark plugs should be changed every 40-60K so if they are massively overdue that could be affecting it. Tons of factors sadly

A bad spring is a fractured or broken spring. If it’s corroded it’s not a bad spring but an advisory that it may break soon. Sounds like the mechanic doesn’t have a clue. If the vehicle fails a “bounce” test the shock is dead. ( press the vehicle down by the front fender/wing ) if the vehicle bounces more than once or twice or doesn’t seem to settle the shock has blown. I wonder if he means the shock is bad. Shocks go bad all the time but for me coil springs can last absolutely years and years and won’t be replaced till they are broken or excessively corroded.

Up to you. Personally I love getting my hands dirty and would jump at the opportunity just to rebuild it to clear my mind and get myself out the house. But if it’s not making any noises I would just count yourself lucky and keep an eye on it. Most likely it may be sightly damaged and may prematurely fail but that could be way in the future.

Customers are too quick to complain and call mechanics scammers. I would assume this is the issue you’re having. They are not completely sure what the problem is and don’t want to deal with a complaint if they are wrong. Vw of that era love killing coil packs. Personally If you were my customer I would go down the route of inspecting spark plugs and possibly replacing coil packs. But a diagnostic should be carried out first. If the eml light is appearing the code should be getting saved so unless they are just cleaning the code and sending you on your way. I suggest taking it to a reputable VW specialist not a backstreet garage. If the specialist says it’s fine I would trust them more than others. Wouldn’t even trust main dealer personally

This is why in my country you need to be trained to work on hybrid and electric systems. If the engine didn’t turn on it will probably be fine and realistically even if the engine did turn on and you noticed in time that short amount of time wouldn’t have damaged the engine. I always recommend learning how to do a service yourself to be honest. Can happily give you tips and advise tools needed, And a couple YouTube videos.

A dying alternator may sometimes work and sometimes not. I had an alternator that would only charge when load was on but would be sitting at 12.4volts without load. Replaced the alternator and now charges properly. That was on a 2009 Audi a6.

Dead cell on the battery. Had a car where you could jumpstart it but if you turned the lights on the vehicle would shut off. Had all the new mechanics trying to sell alternator, imagine their face when a battery fixed the entire issue 😂

I absolutely love vw but the amount of coil packs I have had to put in is ridiculous lol. If it’s a diesel probably a blocked egr or split boost pipe. But definitely needs a diagnostic

Rather used tyres over bald tyres but definitely depends. On my cheap beaters I run part worns sometimes on my daily I run mid range tyres.

You can always make things work but the question is will it cause more problems than it’s worth. My old civic I bodged the maf/map sensor and it caused shit tons of problems. Previous vw I have owned didn’t care tbh. If u can stick the pod onto the oem pipe work ( where the box lid is ) then shouldn’t cause much issue. If you’re changing all the pipe work then u could drill into it but you will be chasing air leaks for ages. You can definitely make it work but won’t be a simple two second install.

Take back to Halfords. As a former colleague of them go straight to hr/customer services. If you just turn up the garages it’s difficult sometimes for them to fit you in but as soon as customer services and hr gets involved suddenly they pull a finger out and will rush you in. Sounds like a poor condition but could be they left something else loose or trapped a wire whilst doing the battery. Without seeing it I can’t tell you exactly the problem sadly it needs to been stripped again realistically.

Could be a few things suggest getting the battery checked first. If the battery is fine then could be the starter motor. Battery is definitely the first step even a basic volt meter would give u a rough guess before throwing money at it