I didn't mean as a training base, I meant for matches at U21/19/18 level, maybe even U16/Victory Shield level. These are, to my knowledge, all still played at club venues.
As for the hotels, I couldn't say definitively, although I certainly think there would be a case for it. If you have conventions that last more than one day then staying on-site would be preferable, and there could well be demand for a few hundred rooms at least, obviously dependent on the size of the conference.
If you are then making money out of the hotel, you could probably afford to put in a cheaper price for the exhibition hall and make that more competitive, thus attracting more business to it. Possibly a bit speculative but sound in theory.
The thing that has set my alarm bells ringing is everyone connected with SISU mentioning CCFC getting access to the matchday revenues, but making no mention of this area of income, which supposedly makes up a majority of the Ricoh's takings.
Knocking the arena down and building more halls would be an option, although one I fear would be frightfully expensive. I think putting a roof on could be a more efficient, although still costly, option. I don't know if the stadium was designed with extra load bearing capacity to take the extra weight or if strengthening would be required, but if you did that, along with something akin to the StadiArena thing Fletcher had, you could keep the large bowl for stadium gigs (both open air summer gigs and closed roof off-season ones assuming you make it acoustically sound), but also split it up for smaller gigs/conventions if necessary. Much more flexible and configurable.