darrenSP - You can easily update the confirmation message from a text editor. 

Below is the text key used for show Thanks for your feedback message. 

page.RatingEnumerationDisplay.feedback.success.title

Studio -> Text Editor -> Community Text 

Lithium uses the text key for each string appearing in the community.  You can search for the string you want to update from there. 

https://community.lithium.com/t5/FAQs-email-templates-and-text/About-text-keys/ta-p/109029

https://community.lithium.com/t5/FAQs-email-templates-and-text/Find-text-keys/ta-p/109027

Regarding the custom component for this, there is no POST API call available for helpfulness, you can only perform GET action. 

https://community.lithium.com/t5/Developer-Documentation/bd-p/dev-doc-portal?section=commv2&collection=tkb_helpfulness_ratings#constraints-message.id

jaikumar1 - 

Here is the script which you need to add on a Mobile Post Page.  You can create a custom component and add it to MobilePostPage.

<script src="https://cloud.tinymce.com/stable/tinymce.min.js"></script>
<script>tinymce.init({ selector:'textarea' });</script>

Selector value should be the id or class of the Textarea on a Mobile Post Page.  e.g .lia-form-body-input.lia-form-type-text

Documentation for rich text editor https://www.tinymce.com/docs/demo/full-featured/

Hi,

the LITHIUM.CommunityJsonObject is available on mobile as well and not limited to desktop. However, it may depend on the specific device supporting Javascript.

Please note that testing the mobile view using the "?device-view=mobile" parameter in the browser will disable Javascript, so the variable will not be available. You will need to either use a mobile device or change the user agent string of the browser (the easiest is test this on safari with the Developer menu).

Hope it helps,

Chiara

jaikumar1-  There is no OOTB component for only register link, However, You can use  <component id="common.widget.user-navigation"/> and hide the extra links using CSS.

For private message you can use below component.
    <component id="notes.widget.summary"/>

It will appear, even if you don't have new message.