[SalesForce] Want to update a Contact Record or Create Lead using Form

Based on what you provided in your question and what I understood you are asking, see below answers. If you can provide more details on your question, I may be able to provide better solutions.

1. Use the Lookup() function to gather these as variables and display the variables. E.g. SET @YourVariable = (Lookup(yourDE,"FieldtoLookup","SubscriberKey",SubscriberKeyValue) and in the HTML, use %%=v(@YourVariable)=%% to display it.
2. Use required="required" or add in some JavaScript validation script into the HTML.
3. AMPScript is a server-side language, it cannot be used to look up info that is in the HTML (e.g. a table). To pull values from tables, try JavaScript or jQuery for this.
4. You can use AttributeValue() if coming from MicrositeURL() or you can do a variable with a lookup, like explained in #1. You would need to pass a parameter in the link, something like ?Email=Sampleemail to use this.
5. You would need to change the POST location or otherwise edit the processing page to use AMPScript to pull gender (e.g. RequestParameter()) and use IF ELSE statement to define content/styling.

Reference:

MicrositeURL() & RequestParameter()

AttributeValue() & v()

Lookup()

There are 2 ways that we use to pass user information to Landing Pages:

Option 1: QueryParameter

In Email:

%%[VAR @email,@name,@landingpage
Set @email = 'email@example.com' 
Set @name = 'Test' 
Set @landingpage = Concat('https://pub.exacttarget.com/landingpage?email=',@email,'&name=',@name)
]%% 
<a href = "%%=RedirectTo(@landingpage)=%%">Click Here</a>

On LandingPage:

<form>
  Name:<br>
  <input type="text" name="name" value="%%=v(QueryParameter('name'))=%%"><br>
  Email:<br>
  <input type="text" name="email" value="%%=v(QueryParameter('email'))=%%">
</form>

Option 2: LookupRows & DataView

In Email:

%%[VAR @subscriberkey,@landingpage
Set @subscriberkey= '123456789' 
Set @landingpage = Concat('https://pub.exacttarget.com/landingpage?key=',@subscriberkey)
]%% 
<a href = "%%=RedirectTo(@landingpage)=%%">Click Here</a>

On LandingPage:

%%[
var @key, @email, @name
set @key = QueryParameter("key")
set @email = FIELD(ROW(LookupRows("_Subscribers","SubscriberKey",@key),1), "EmailAddress")
set @name = FIELD(ROW(LookupRows("Example_Customer_Data_DE","SubscriberKey",@key),1), "Name")
]%%
<form>
  Name:<br>
  <input type="text" name="name" value="%%=v(@name)=%%"><br>
  Email:<br>
  <input type="text" name="email" value="%%=v(@email)=%%">
</form>

Note that both of these methods are taking values from AMPScript at the time of send, and appending them to the destination URL string. For this reason, you may need some security/hashing/encoding to prevent malicious attempts to access your customer data via the form.

I hope that helps.