> Does IF Metall already represent all of Tesla's employees?

No, but Sweden has mainly unions based on industries (like IF Metall represents workers in industrial and metal-sector, you have Transport that represents workers in transport industry, Unionen of white-collar trade workers, etc). There might even be no workers in the specific union at the specific workplace and they might still go on strike since they are the union you are supposed to negotiate with.
There are requirements of negotiation but no requirements of results from that, which is the cause of the current situation at Tesla. Tesla "negotiated" but flatly refused any outcome involving the union so they fulfilled the lawful requirements. Now the Union uses the tools they have in place when the negotiations ends badly so everything is going as expected.

>Does that mean there are no active Tesla services in Sweden during the strike?

Like I wrote above, Tesla might have all services running if they have workers doing it, but chances are at least some are members of IF Metall. There is also the effect of sympthy strikes from various other unions:
https://www.ifmetall.se/aktuellt/tesla/darfor-tvingas-if-met...
This includes transport (loading/unloading in harbors, transport of goods, etc), Electricians (yep, you can guess it, practically everything involving electricity), Builders (maintenance and construction of buildings), service and communication (This is postal workers refusing delivery of post/goods to/from Tesla), etc. The effect goes beyond Teslas services and will extend to all kinds of infrastructure and other parts of society unless Tesla manage to get their own self-sufficient (and probably totally isolated) mini-society without any needs from other parties.

So the more Tesla refuses to find a deal, the more the unions can escalate and it will get quite tricky to find laborers who want to deal with you. Other companies might refuse to deal with you with risk having the strike extend to you to if this goes far enough.

> If union membership is really the way of life in Sweden, is there anything preventing the government from a legislative solution?

It is really complicated, but the main thing preventing government going in with a legislative solution is that no-one (neither the industry or the laborers) wants it. It has been a solution that the union(s) and industry can solve this by themself and as long as they can, the government will stay out of it. This is kind of unique relaxed solution that puts a lot of responsibility on both the industry and the unions to sort things out and has worked well so far. Playing hard-ball like Elon will probably not end well for Tesla in Sweden (possibly Europe) if this continues since the system is kinda built on being mature and able to negotiate. My way or the highway attitudes are not well regarded.

>Just mandate collective agreements?

But what collective agreement? The system in Sweden is built on negotiation between the involved parties and finding some kind of common and sensible ground. There is really no easy top-down fix for that if one party refuses to play ball. Either the other party need to fight back or the whole system collapses and we risk ending up with government moving in, which neither party wants.