I can't speak for the septic tank issue, but about the faux seawater/lye solution I can share my experience. I always freeze my liquid, then use the ice. I have regular and mini ice cube trays so weighing isn't an issue, mostly I do it to keep the temp low and reduce fumes and scorching (I mostly use milks) because I have a very small work space with barely any ventilation. When I add lye to ice it typically dissolves slowly, and when I used the brine/faux seawater, the mixture was cloudy and had some pieces that took forever to melt, but you can always pour through a mesh strainer to catch the last few bits that won't dissolve. But for me, it takes 5-10 minutes for all the lye (I use flakes, bought from a soap supplier) to dissolve depending on my concentration, what liquid I'm using, etc. I did use lye from the Home Depot once and those were thicker bits and took FOREVER to dissolve, so I went back to getting it from a soap supplier (usually Brambleberry).Well it's 11 o'clock at night I have had along day because every time I went to measure out my ingredients someone came over Or I had to run over to my son's school to fix his schedule for next year today was the last day of school so I had to go. Anyways I have everything ready and measured out to start soaping and now my lye will not dissolve into the faux sea water I don't understand what's happening. The other problem I have never thought of is how will I dispose of this lye now we have a septic tank and we aren't supposed to put lye down the drain what should I do? Oh I was so excited this morning to make this soap and now I feel tired and bummed right out we only have one kinda lye intown the kind that is in pebbles. So I don't know if buying a new batch of lye will help. Well I will have to figure this out tomorrow. Could mixing my lye to slow with really small amounts of lye at a time to cut back on fumes be the problem I usually do this but never had a problem before.
Good luck on your soaping journey! And remember, it's not a failure, but a learning experience!