Is it impossible to change group announcements?

It means the announcement you made contains words violating the group rules. Solutions to that are as follows:
1.Build a new group of your own;
2.Make an announcement in the group by use of the words you want;
3.Copy the sentence one after the other and then enter “send”;
4.If a sentence cannot be sent out, it means something wrong with this sentence;
5.Then modify this sentence.

Is it impossible to change the icon and name of the group?

First check if the group is certified. For certified ones, both their icons and names are unable to be altered.

How to generate short links and QR codes?

短链接请访问如果想更稳定建议使用百度短网址需要进行企业认证二维码请访问草料二维码
For short links, please visit: http://tool.chinaz.com/tools/dwz.aspx,if you want a stable link, suggestion for you is to use the Baidu short website, with the need of corporate certification.
For the QR code, please visit the straw QR code.

How should I do if the QR code cannot be identified (the one on a poster)?

Inspect according to the steps as follows:
1.Check whether the network is in good condition.
2.Check if the QR code on the poster is too small. Being too small may result in non-recognition (because QQ can compress the picture).

Tips: Before QR code generation, the original link can be cut to a short website and then it can be generated into a QR code. In this case, the QR code looks much simpler. For generating a short website, see below.