A: According to the Architect, he is unable to physically approach the Mother thanks to enchantments in place. That's why he needed the Warden's help in the first place.
B: You're assuming that he wouldn't consider Seranni a necessary sacrifice to his greater agenda. She was his follower, she may even be willing to give her life for him (And really, she isn't actually very valuable to him in and of herself).
C: Faking his death means that nobody will come looking for him, no Wardens hunting the Deeproads for his followers, and he'll be able to continue operating in complete secrecy.
D: You're assuming he sees the Wardens as enemies. Even if so, if he leaves them alive they serve as witnesses to his apparent demise. If he just killed them, the Wardens at Vigil's Keep would still report his existence to the Wardens at Weissaupt and the Mother would remain a threat. We're back to status quo and the Mother now has time to regroup.
Then again, there's also the scenario where the Architect is not aware of his body jumping ability (If he hasn't died before) and simply leapt to the living tainted individual most closely connected to him and willing to help him through the taint when he was killed: Seranni.
A:He says he can't approach her not because any enchantments only because her children protect her (yes those small darkspawn)... ,what in first place wouldn't matter if he was immortal.
B:Necessary and pointless sacrifice are 2 different things.
C:In First place the warden knows about his followers and his plans ,so even if s/he thinks architect is dead hunting still would be there , also the warden and companions present in mother lair were only one who knew what was going on as architect explained that and what he plans to do.Grey wardens know about architect at least for 20 years before daa ,plus there would be no problem at all with faking his death by just leaving his body so yeah see pointless sacrifice part..
D:Don't even bother with that ,the warden is his enemy as i said s/he oppose his plans in first place ,so yes the warden is his enemy in that scenario ,unless you want to tell me that person that oppose your plans and killed you is your friend.Wardens knew about architect long before daa in first place ,second simple he could just leave his corpse with same effect , third as i said if he was immortal the mother wouldn't be problem.
Also in that scenario he either would be just dead as he wouldn't be able use that ability as he wouldn't know about it ,or if it is automatic (and it rather it isn't) he would just jump into closest warden be it the warden or his companion not "person that trusts him".
Not to mention that there is ton of stuff that show architect don't have abilities that corypheus had ,like for example ability to influence and control grey wardens.