The reason you are getting nothing for relatedToType="Contact" is because Contacts, Leads, and Users can't be used in relatedTo (equivalent to the WhatId)
See Apex Developer Guide outbound Apex email method setWhatId
The value must be one of the following types:
Account Asset Campaign Case Contract Opportunity Order Product
There are couple of ways you can do this.
Using Queues. Create Queues respective to Distributor Account and assign the Partner Users on that Account to the Queue. Using assignment rules, just assign the Lead to the Queue, so that the members of the Queue get access to the Lead records. This way you will also get the Queue List View available to your Users.
I was facing the same issue and realized there is no way to change/translate "Choose File" label unless you change the browser language itself however in Salesforce mostly we prefer to use Labels and picklist translations to make Salesforce UI user friendly so that unexpected translation doesn't spoils the user experience.
Here is the way we can translate, ...
I don't think we can raise a case from Partner developer org. I tried in past and didn't get success.
You need the environment hub org to raise the case from there you can share details of this Org.
Also if you don't have premier support salesforce won't help you much in that case.
From partner org > login > go to home > There you find button Support and ...
SessionId or accesstoken in SF expire if there is inactivity for few minutes / hours based on your profile settings. You have 2 options
In profile setting for the user, change the default Session Logout to be longer
Write a poller in your implementation to automatically call any endpoint. It will call as activity for the given period, it will make sure the ...