A good antivirus will detect all forms of malware. It is not advisable to run more than one such app, as they can conflict with each other and give false positive reports.
Not necessarily, no antivirus product is tasked to detect malware unless it was intended to be a full-blown complete detect-all package. May companies offer different tiers of protection.
MSE is a good example of this, it uses 2 engines to deal with viruses, worms, trojans, etc. then a different engine (Defender) to deal with malware. Does it do a good job? AFAIK, I haven't heard of a single case where someone actually got a real genuine virus, a trojan maybe. The problem with the detect-all type of apps is while it's nice that it can do everything, having realtime protection is a burden on any machine. You can run MSE in the background while using SAS to manually perform a scan then follow up with MWB for a secondary scan.
SAS and MWB... both free apps, are meant to be used on-demand. There's very little issue if any about conflicting reports. Antivirus scanners, especially the top products have issues with false positives, whereas the more simple products don't due to the differences in heuristic scanning quality.
My suggestion to users is no different from how I'd do it for my own personal machine.
1. Install a lightweight AV product. For Windows 7 users, don't worry about buying the best detection product out there because most of the large detection products detect stuff that no longer affects Windows 7.
2. Download Malwarebytes and SAS, the free versions are on-demand protection which means you click on a few buttons for them to work. The full-paid versions offer realtime scanning but I'd suggest against it because it may/will slow down your machine.
3. SAS (Superantispyware) has a portable version, I'd suggest using that instead of the regular installer. Run the .exe file then update it before you start your scan. Nothing in the registry to worry about, no uninstall hassles for when you want to delete it.