I currently admin 278 email accounts (and counting) across 109 domains (and counting) that consume 58GB of storage (and counting). Ran my own RC installation for years (had melanie2 mobile skin working well) until a hosting company move yielded a database incompatibility that broke address book, and a later host company’s internal update killed login. With your Reseller-150 product… can the advertised send per hour thresholds be raised (for a fee), for any account that wants to bulk mail, we employ mailgun - so bulk mailing will not be performed through this package, just a lot of users can equal a lot of sends. if we outgrow the package and have to upgrade, will that affect the programmed-in server dns settings for the email accounts? while my domains will be at a third-party registrar, do you provide for auto-config of the email accounts served - into outlook, apple mail, etc (with proper dns settings in place)? is your hosted version of roundcube the latest (1.4.7) WITH the mobile responsiveness? are there server-side spam filtering controls per email account extended to the roundcube UI? or are the filters only globally applied to the entire package (can each user somehow administer their own spam, block, and allow list)? I see that other [than roundcube] webmail interfaces are available, can those be disabled as an option on the front-facing custom branded user signin of the package (i.e.: do roundcube only - from an end-user support standpoint)? are there any storage limits per email account? can soft-limits be set per email account within a package? Can I see a demo of the roundcube installation that you employ to see what settings and features exist? can feature upgrades to the roundcube installation be requested (while understandably all will not be fulfillable)? Do you provide for group forwarder/alias(es) of an email account?
If you use MailGun the limits won’t apply as you noted - the mail won’t be going through this package. The advertised sends per hour are per email account, so no, raising the limits is not really offered as they are generous as it is. (300/hour)
If you outgrow the 150gb you will need to order a second package and migrate accounts accordingly. IMHO it might be a good idea to limit the number of emails/domains and just order a second package when you hit 75gb - may be simpler than migrating later.
No, it is not provided by MXroute but is easy to host somewhere. I paid a Direct Admin dev to set it up for one of my clients … you likely can set it up yourself. (I am not a web dev)
Yes.
Yes, and yes.
Disabled, no. You could simply not tell clients that they exist. This is not cPanel where you log in and choose 1 of 3 different webmail clients.
Whatever you configure them to be when you create packages. Anywhere from 50mb to 150gb. 
Not sure I understand the question.
Probably not. Requested, yes … but fulfilled depends on the panel developers.
Aliases / forwarders are available, yes. Not certain your meaning with ‘group’ though.
Did I miss anything?? I am a fellow reseller so the answers are from my perspective and may differ slightly from @Jarland’s perspective.
It may be worth your time to explore the Tutorials that are available.
-firstly, thanks! you did answer most of the questions.
-what i meant by group forwarder is such as:
acct1@somedomain receives and then auto forwards a copy of its inbound messages on to a list of recipients. i.e.: forwards to acct2@somedomain , acct3@somedomain , and acct4@somedomain . some cpanel hosting I have only does 1-to-1 forwarding and doesn’t provide for 1-to-many forwarding (a group forwarder). I would like group forwarding, even if it is achieved as multiple 1-to-1 entries/aliases.
-can each email account have a unique storage size limit set within the package, and can that limit be adjusted without having to delete and re-setup the account (soft-limit/soft-cap)?
-is there a cname entry per webmail interface? i.e.: i can set webmail.mydomain.gtld to only go to the roundcube installation? otherwise what is the UX roadmap to arrive upon served webmail UI?
-so you stated that users can administer their own filtering via roundcube UI. is that managesieve filter client-side (only runs when webmail is opened) or server-side (filters even when using third-party app to operate email account, webmail not active)?
-and fwiw i believe i was in RC1.16LTS heavily modified, but additionally had the plugins: vcard_attachments, contextmenu, newmail_notifier, and ident_switch (some of which may now be part of 1.4.7 default featureset). which is why i would like to test drive the webmail client to know what to expect vs past provided end-user experience.
It is multiple 1-to-1 entries.
On the reseller level you will create packages which will determine how much storage is available to an account. On the account level the ‘client’ will create users and each user created has their own limit set at the time of creation - which can be modified at any time (or be set to unlimited - subject to total storage of the account).
Here are mine as an example:
Roundcube - https://webmail.anydomain.email/
Rainloop - https://webmail.anydomain.email/rainloop/
Crossbox - https://email.anydomain.net/ (I haven’t set it up on the .email TLD)
So the webmail subdomain can be used EITHER for RC/RL or Crossbox. My preference is to use a different subdomain for Crossbox to allow the most flexibility - just in case there is an outage with Crossbox, a client could use RC/RL instead.
Server-side. You can manage them with sieve … sorry I am weak in this area, so don’t know much about the subject.
Unfortunately, Jarland does not offer trials … it is what it is advertised to be.