Some tournaments do require late entrants to wait for the BB in blind games. For example, this excerpt is from the WSOP's Dealer's Guide in the multi-table tournament section:
LATE REGISTRANTS
In games with blinds, a player who enters during the first round of play will receive no
penalty so long as the blinds have not passed their position.
Players who enter after the first round of blinds must wait until their first opportunity to post
the Big Blind to begin play [...]
Subsequently, it was clarified that new players to the tournament could "post" rather than wait for the BB, and start playing right away. It is a bit strange because I am accustomed to seeing players post or wait only in cash games, but I guess it is workable. Presumably, by forcing new incoming players to post or wait, this would prevent new players from dodging the big blind and coming in for "free" after the button passes his seat. However, I did note in at least one instance that dealers were asking players who were late to the table but whose stack was already put out and being blinded off to also post; this seemed odd to me since the player had already paid his blinds for that round, so posting effectively penalizes him twice. Anyways, I admit I don't fully understand how this policy was intended to be enforced.
To address the original question, there is no specific rule about seating alternates coming into a table with in the TDA rules or RROP. However, there is a provision in RROP (Tournaments, #16) that states that new players to a table as a result of balancing tables will be dealt in immediately unless they are in the SB or button position in which case they must wait until the button passes. It would seem that it would be fair to apply the same principles to this situation, and I would tend to have the incoming new alternate wait for the button to pass his position in both situations.
That being said, there is a debate going on in another thread regarding balancing tables that has some similarities to this situation, and I can see a case being made for having the alternate take a single Big Blind right away in the first situation that gets the new player playing right away. Who knows... In the future, it would not surprise me at all if some TDs decided to even allow a player to "buy the button" as one might do in a cash game in tournaments, particularly in your second example by having the alternate sit and post BB and SB in order to play right away! For now though, I suspect that it is more "standard" to have the incoming new players simply wait for the button to pass in tournaments.