If, out of curiosity, Flowey was to absorb Aliza's soul, would he transform into his Asriel Dreemurr form? And if he did, would he be capable of feeling love like in classic UT?

He wouldn’t! Two of the six original souls have died in his care; adding one more, no matter how powerful, would still only leave him with five, and a full seven is required for ascension to true godhood.