Amongst WeChat's many local competitors were equivalent messaging apps including China Mobile's "Feixin" messaging app and China Telecom/Netease's "Yixin". Both competitors, as officially published apps from the telcos themselves, had the ability to leverage free SMS messaging, an ability that WeChat did not have access to. WeChat is considered a tangential competitor to the telcos.
WeChat doesn't block or QoS other IM products. That would be illegal and, frankly, a PR fiasco in addition to a great way to lose user trust.
3) It's against WeChat values to push mobile phone makers, distributors, or ROM publishers to preinstall WeChat for pay. In addition to being against WeChat values, it's also a big hassle as preinstalls require careful version management. Due to the popularity of the app and the relatively high cost of data for many low-end users, many of these distributors have voluntarily preinstalled certain popular apps including WeChat as a convenience to their consumers. WeChat did not "negotiate" to keep WeChat app in memory -- I'm not sure where you're information comes from. For users within China where Google Cloud Messaging is not an option, a background process for Android continues running to receive push notifications from a notifications server. WeChat works with various partners including "phone security" apps to make sure that this background process isn't being unnecessarily killed by an overly aggressive memory manager process which would result in not receiving notifications. Outside of China, WeChat uses GCM for push notifications for Android devices. This is a common requirement for apps within China since GCM is not available, but because some apps have lower engagement they are more likely to have their notifications background process killed by aggressive memory managers.
4) WeChat for Android uses the X5 kernel, a branch of Webkit (not Chromium) and largely initially used because certain security vulnerabilities on older versions of Android system browsers (including things like SSL-vulnerabilities) made it impossible to safely deploy properly encrypted communications and transactions in webviews without making sure there a secure web rendering kernel.
iOS versions of WeChat, of course, use the default in-app Safari for webviews because it's considered secure.
5) WeChat blocks certain URLs from appearing in webviews because a) they contain malicious code and are unsafe to users or b) too many fraudsters were using certain domains to host spammy marketing content and would leverage WeChat to spam out links to these pages to make money or c) local legal requirements require blocking of certain content.
I understand that it's sometimes difficult to get good information on a product that's very popular in China, but not in Western markets. That's why it's so valuable that the YC team has spent their time to help other founders understand some of the underlying dynamics for WeChat's successful product strategy. Misinformation, therefore, is not valuable in helping the YC team achieve this goal.