I would say if BT and WU are fixed I would have some leeway on naps or at least one of them. If you have set WU time and capped naps then every sleep might be disturbed and doesn't give LO a chance to catch up on any lost sleep or additional tiredness. By additional tiredness I mean if LO is teething, ill, or going through a developmental phase they can be extra tired, capping every sleep could result in some OT. There may also be some changes due to various activities, if they are more physically or mentally stimulated one day this could lead to a shorter (due to OS, over stimulation) nap but not fully rested or could lead to a longer nap due to the additional work they have done so more rest is needed. Equally a less challenging morning could result in a short nap due to less stimulation (or US under stimulation) so LO is less tired. This could explain the 1-3hr difference in that first nap.
I would perhaps have a conversation with your mum where you try to come to some compromise, that perhaps 2hr-2.5hr is long enough to keep the rest of the naps on track and perhaps suggest a longer/shorter A time following the nap dependent on how long the nap was.
