How is 80 mage and range not profitable? You will be doing upwards of 15 kills an hour if you know what you're doing resulting in 1.5mil~ an hour profit. Zulrah isn't hard, the only time you get damaged is when venomed/clouds when dodging melee form, or zulrah hits a range in mage form which blowpipe spec heals. You'll only really need 2-3 food a fight and 60~ prayer points.
I agree on the 45 prayer part.
Optional points you made :
1). waste of inv space + illogical if you're going to be botting and waste of time for quests
2). waste of time as same thing as above and to efficiently do it you need the max house pool so you can do a lot of kills before banking which would require high construction, much rather charter ships + 1 energy pot.
4). You're contradicting yourself by saying void range switch and a serp helm in the next point, you'll still need anti venom.
3). Void mage is useless at zulrah and to do zulrah well a 4 way switch isn't enough, you need 8 way switches.
Also to add to what you said of course he can make a 90 construction, 99 hp, 99 def, 99 range and mage with 99 agility and 250 qp but that's a big time waste for him to go 51-71 agility or to 76 agility and complete other quests when he will end up getting a few mil extra for that, not worth his time.