https://www.henleyglobal.com/passport-index/ranking
2. Yes - an example I travel to frequently is Vietnam - it is visa free for Italy but not for USA (former American Italian here)
3. Generally no - USA is the only major country that taxes based on citizen and not by residence or territorially
4. Embassies very rarely help anyone “when something bad happens” - in the event of a large natural or political disaster they might help. Most (including the USA) have a policy of not being able to help you in most situations beyond giving you the contact of a lawyer, doctor etc.
5. Most of the time no, but some places do prohibit citizens from having multiple citizenships or acquiring new ones: iirc Singapore is one of those.
Wrote about my citizenship changing adventures here since you seem interested! https://larrysalibra.com/goodbye-usa/