This is in California... A couple problems are these. Plumbing needs to be replaced because it backs up in places you don't want it to... There is mold under a sink... The heater stopped working...

I have tried to fix the plumbing. Had a professional plumber go out twice in a week. (I have receipts)
I returned myself to do more work and they refused to let me in to fix the immediate problems... We had a brief yelling match about me not doing any work on the house, even though that's why I was there... She is trying to call me a slumlord...

Other than the problems I went to fix, they have let trash build up on the side of the house, which I took pictures of...

Is there anything I can do? I know there is eviction, and I'm starting that, but is there something more immediate I can do to stop from being labeled a slumlord? Like have a police officer come to the house with me so I can do some work?

Police likely won't get involved as it is a civil matter. Don't you have keys? Inform the tenant that you and workpeople will be at the property at x time on x date. If they have changed the locks and not given you a key, then bring a locksmith (and proof you own the property) to let you in and change the locks and give you keys (which you give to the tenants).

Your lease and CA allows you access to make repairs, especially emergency repairs (which the creation of mold would qualify).

If the tenant is there and refuses to let you in, you can try calling the police, not sure if they will do anything though.

As far as being branded a slumlord, you may be able to sue the tenant for defamation. Depending on how loudly they are yelling slumlord.