Oscar was flown to LMU and the coaches were chasing him very hard for a Div 1 scholarship. I don't think it's accurate to imply that there was no interest in his future at all. At the time, I don't believe that Darnell Mee approved of Smyth trying to get him to stay in Adelaide, but you are correct in that Oscar and Jacob were shown appropriate interest by the club - solid contracts, good court opportunities, etc.
I just don't know if it's been all that great since then. e.g., you've made no comment on offering Ingles a spot in the thirteen and a sub-rookie wage? For a guy likened to Kukoc by Goorjian and touted as having potential near that of Bogut and Newley by his AIS coach, I don't think that's enough. I wouldn't have a problem with losing that type of talent to a better financial offer interstate if I knew that the non-financial elements of the offer were substantial.
Time will tell if Holmes and Forman improve further elsewhere, you are definitely correct. I just know for a fact that Forman left specifically to pursue a greater emphasis on individual development because that's what he needs. The difference between what the Sixers were offering and what he was asking for was $5,000 - money wasn't a big motivating factor, he couldn't even be coaxed over to NZ in the off-season last year.
You could fit those players under the cap and give them minutes.
Maher, Newley, Hill, Forman, Ingles, Holmes, Rogers - approx. $645k even giving Maher max, Forman what he wanted this year, Ingles double what the Sixers offered him, Holmes a $30k upgrade and so on. Even lose Rogers for an import at a similar price and you've still got $130k for two scrubs and $80k on a role-playing import or Aussie, maybe a point guard, a Kendall, a Luke Martin for less than that, etc.
Signing players before they get bonus offers from elsewhere would go a long way to keeping a roster like that under the cap. Play Maher for 35 minutes, Newley for 35, Hill for 20, Forman 30, Holmes 35, Ingles 10-15, one import 35, another for 25. Could even have 15 minutes spare to blood the scrubs. Obviously that's all theoretically, but I don't think it's unfeasible.