Let's compare the off the cuff, tossed off sentence you received
Como tenedor mayoritario creo debes proponer tu sobre como disponer de esto
with the same sentence with the accent marks, an optional comma and an optional "que" included:
Como tenedor mayoritario, creo que debes proponer tú sobre cómo disponer de esto.
What this person was thinking was basically
Como tenedor mayoritario, creo que tú debes hacer una propuesta sobre cómo disponer de esto.
Como tenedor mayoritario, creo que debes proponer algo tú sobre cómo disponer de esto.
It's common to put the "tú" after the verb for emphasis. Also, we usually use "proponer" as a transitive verb (with an object), but it's also possible to use it without an object in a more conceptual way. These factors might have added to your discomfort with the sentence as written.
I'll confess that when I first looked at the emailed sentence, my first interpretation of "tu sobre" was "your envelope," which is nonsense in this context, of course. But native speakers of Spanish, which I am not, are experienced at intuiting where to mentally place omitted accents. The sloppy leaving out of accents is extremely common and everybody but me seems to deal with this quite well.
What made you most uncomfortable was the use of the word sobre. It's common in the construction "hacer una propuesta sobre cómo hacer tal cosa" (make a proposal about how to do such and so). Technically, your correspondent didn't misuse "sobre." It's equivalent to "acerca de." Maybe this variant would be clearer for you:
Como tenedor mayoritario, creo que debes ofrecer una propuesta tú acerca de cómo disponer de esto.
I think your correspondent was just extrapolating from that. Yes, it's a sloppily written sentence, in that the author wrote it and hit "send" immediately, but technically the original sentence would not be grammatically incorrect if one projects the implied accents. It's best to draft an email and come back to it at least five minutes later before hitting "send," and in this case would probably have resulted in a clearer sentence. For example, it's okay to omit "que" but including it helps the reader parse the sentence. Here's one possible revision:
¿Cómo debemos disponer de esto? Como tenedor mayoritario, creo que te toca a ti proponer algo.
(Functional translation: How should we deal with this? I think you, as majority holder, get to decide how to deal with this.)
It's very common to send one's first draft immediately but in the long run I don't think it's the most efficient way to work.