Q: Can tribal casinos manipulate the program at will?
A: If they are self regulated and have the technology, I would say YES
Do they? I dunno
At least in Oregon, the lotto machines in bars, etc. are like the 'scratch ticket' you speak of and the tribal games are something else altogether afaik. If this is the case in washington, it is two seperate questions.
I've experienced the feeling of what you are saying about high and low volume periods in tribal casinos, and also find it funny that every fifteen minutes or so 'the voice' calls out another $700, or 2500 or thousand dollar winner, but I have never seen one hit

and it's funny how more often than not when they have a drawing out of tens of thousands of entries, somebody who had to be present to win, does. hmm,- not in reno, not in vegas.
I think I pay more attention to this kind of thing because I believe they are self-regulated, I don't know what the house edge is, and I really don't trust them - any more than I would trust a tax collector who only had to answer to his boss. But I play anyway.
It may be totally fair, if that wasn't a possibility I wouldn't play, and I doubt you or your friends would either
If they can, and have, they probably do. What can you do about it? Play online or fly to reno or vegas for a two-nighter.... cheapest deals are southwest from there, dirt cheap. But if you don't mind donating through a casino, where at least the tribe is in control of the proceeds, play on