Is Lavellan being mortal really such a big issue? Sure, she might die and he'd live on. But presumably Solas has had to deal with this before. It doesn't make it easy, it doesn't make him care any less. It would be a tragedy, and it would hurt. But people don't tend to just exist in constant agony when a loved one dies. Life goes on. You aren't expected to just die because the love of your life dies.
I'm not a really big fan of the idea that the only way they can be together is for one of them to give up mortality/immortality. Love is a great, wonderful thing! But it isn't the be all end all of someone's life.
I always found the whole watching someone else get old but you stay the same age thing a little... odd/uncomfortable. It is by no means a requirement to have someone give up mortality/immortality but Lavellan is still elven. As far as I remember there is some dispute over whether the Dalish have longer lifespans than other elves due to Lanaya's statement that they have lengthy lifespans, Master Ilen's story about his parentage, and some claims that Dalish have human lifespans. It might just be due to some lore being changed or tweaked from what it originally was, or it might be due to some rare cases, but if such was true it brings up the problem of the Dalish forgetting so much lore in so few generations.
ramble ramble, back to the point. There isn't confirmation about how long Dalish currently live, and past elves had been immortal. It's plausible for Lavellan to, with some help, figure out how to gain back the immortality the elves had lost. There's also the question of Solas' lifespan. Is he actually immortal or does he have a way in which he sustains himself that we do not understand yet? As far as we know Solas could just have an extremely extended lifespan as he is afraid of dying alone. The wording of his fear just seems a bit odd to me if he were truly immortal. He could always make some horcruxes to avoid most deaths, so I'd argue his fear would be more along the lines of being the last one left, watching everyone else die or failing to correct his mistakes, as death itself would be a secondary concern.
Basically, I think there should be, and it is plausible for there to be, an option of sorts to match lifespans but the option of keeping current lifespans should be there as well.