"When I run DISCONNECT.ME to search, it's redirected to DuckDuckGo"
Disconnect.me is redirecting to DuckDuckGo while they work on restoring their Google search syndication, it was falling back to Bing syndication which was providing less than helpful results so it will now redirect to DuckDuckGo.
"if I enter an onion address, I receive a 404 error."
Restart Tor until it works. You probably got a bad exit node, with bad traffic. Restarting Tor will get you new circuit
Please note some sites will intentionally block all Tor traffic, in which case there is nothing you can do
Can specific exit-nodes be "turned off"?
Can groups of exit nodes be "turned off" based on top-domain and/or on location (ie. country)?
You can add an ExcludeNodes to your torrc config file.
From the manpage:
ExcludeNodes node,node,... A list of identity fingerprints, country
codes, and address patterns of nodes to avoid when building a circuit.
Malwarebytes is blocking Tor - it's a known issue. If I got you right - you have two antiviruses on one PC, it's a bad idea nowdays: I remember 1994 - yes, then it made some sense, but not now. Some network filtering is remaining when the AV is deactivated, and - if your host is also Windows - it can activate windows defender+firewall when disabling other ...
You can try meek transport - it seems that you're experiencing a SSL/TLS based intrusion tactiks from your ISP, so use that particular transport: it is specifically designed to fight with such type of problem