Can I delete these files from my Smartphone to save space?

There are many files which are created for backup and other functions which a program might be using for knowing certain things in historical order or keeping a log of things happening in the mobile device. These files are of no use for us. May be a professional might use them for knowing the details of the system and recovering the deleted data for some specific use. We do not require them as we are neither able to do that type of professional work nor interested in retrieving the deleted data. So best thing is delete all these files and space will be created in your device. I am regularly deleting them. One thing to note in this matter is that even if you delete them some programs or system would again start making them with newer data and after some time their size would be considerable to create problem of storage in the mobile. So, what I am doing is that I am routinely deleting them. The WhatsApp one which you have mentioned contains some data which only WhatsApp might be using so almost every week I am erasing that one.

What I would suggest as a safety measure is that whatever important data is there in your mobile device please shift or move it to SD card under some folders for category purpose and after that delete these unwanted files as mentioned in your query. Keeping all the data in SD card is a very good method to reduce the crowding in main phone internal storage and speeds up the phone considerably. Once a SD card is full you can remove it, tag it, and put a new SD card in the mobile device. Please give the feedback on this through ISC message system or any other way you like so that we can improvise further using this method.

You can delete all the files without any issue. I cannot comment on LOST.dir file but the rest I have deleted earlier from my mobile without any issue. I have even deleted LOST.dir but have not faced any issue on my mobile to date.

The storage/Whatsapp/database/msg store.db.crypt12 files are a daily backup of your WhatsApp chat out of which you can delete all except the latest one. Whatsapp takes a daily backup and after every few days if you would check the internal storage you will find them there. So, go ahead and delete them without a second thought.

The opus file extension is an audio recording, so if you have an audio recording feature turned on then you can disable it to stop such files from getting created again. Sometimes this extension is also a type of ringtone which plays with a particular media player only.

Regarding similar files of 25MB size maybe the old Wahtsapp backup must that may be left on your phone, in case you have installed the Whatsapp again. So, it is completely safe to remove them too.

You can refer to this thread wherein a member faced a somewhat similar issue:
Regarding my phone memory and software updation problem

The last option is to take a backup and reset the phone and get a memory card installed to get more space on your smartphone.