1) Peter Blaise is quite correct: the answer for “How Do You Know ?” is VirusTotal. Once you have a bit of experience using it, you get to see certain things, like which AV scanner companies are the longterm, major players with a lot of established street cred, vs. which are the no-name wannabees, who you can disregard. Even then, there are certain fairly common things that can trigger false positives. Look up the longstanding complaints of highly respected vendor NirSoft (Utilities), for example. So, you look for patterns and the number of flagged “hits” at VT.
2) This is hardly a new category; no one has just invented the wheel here. Hiren’s (I’m referring to the “uncompromised” edition here) was long the gold standard in this category, and it worked either from bootable DVD or flash drive, but it has not been updated for a couple years now; there’s no telling when an updated edition may turn up. Then there is a guy on FB and with his own web pages, goes by the name of “Gandalf”, who has been putting out his own fully loaded Win PE collection for the last few years, and it did have some merit. But he began charging for it, and became something of a pain to deal with. (As Gandalf himself had pointed out, if you are technically adept there are guides and resources online for creating your own version, stocked with the utility / rescue / repair tools that you prefer.) The remaining best alternative is still free, the collection from Sergei Strelec, which gets an updated edition at least twice a year. At least, such was the case prior to the pandemic. Among other fixes or improvements, later editions may address certain changes that have come across in recent Windows OS. That said, the underlying bootable basis for these collections seems to be IsoLinux — which does not seem to present any real problems or complications for the user. These are not the only choices you could find out there, but the Strelec collection is quite good and has become my preferred option.
3) UEFI: what a colossal pain in the butt ! It’s supposed security benefits mean nothing re my personal computer use. I’m not as well-versed in this as I should be, but so far I have found no way to get around the boot-blocking aspects of UEFI from external media, other than to ignore some dire warnings from Dell or whoever built your rig, and to temporarily reset your BIOS settings to “Legacy Mode.” Then, do what you need to do with your emergency boot & fix solution, changing back to UEFI settings once done. I would really love for there to be some very convenient, elegant workaround to this. (I would have thought that Acronis and several other imaging / backup software vendors must have finessed answers to this . . . ?) And I’ve read claims that IT departments have developed simple scripts to automate this multi-step nuisance, but when I request that they make such a script available, the response has so far been a deafening silence. If someone had a safe, reliable, manufacturer-versatile script like this that was proven to work, I might even pay for it !