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Ryanair is indeed the issue. BUT ... they can't be discussed as an inconvenience / afterthought. They're on course to establish themselves as by far MAN's largest carrier by passenger throughput. It's not just the based fleet; they schedule a substantial number of visits by overseas-based aircraft as well, and these appear between the waves of MAN-based units. They usually fly full or close to full (C-19 excepted) - upto 195 seats per movement. As we emerge from C-19, Ryanair is the ONLY carrier which has returned to anything close to full strength at MAN. They're dominant, and it makes sense for them to push home a slot and terminal access land-grab whilst the going is good (from their perspective). Building up capacity in this dire market won't come without financial pain ... it is a big investment for the future, when additional grandfathered slots at attractive times will reap rewards for the long-term. Further to this, FlyBe 2.0 still holds the original FlyBe slot portfolio at MAN, along with access to scarce T3 capacity. Their slot horde is significantly larger than that of TUI, a major based carrier. If they genuinely plan to use all those slots, good luck to them. But does anyone really expect that? If not, Ryanair is the company best placed to benefit. And MAN would be very wise to accommodate them. Especially as Ryanair is simultaneously MAG's core partner at STN and a substantial passenger operation at EMA.
Your premise puts ONEWORLD first. They're a valued customer at MAN, but their ops are way, way behind Ryanair and EasyJet in importance. BA Mainline operates a Shuttle to LHR. A320 family aircraft around six times per day. Sun-Air operates BA-branded J328's on a niche programme to Scandinavia (currently C-19 suspended). BA Cityflyer holds historic slots for a weekend leisure programme in S22, but may not return in reality. American has exited MAN, and any return looks like a single daily short-season operation at best. Iberia Express is less than daily to MAD. Vueling is upto ten flights per week to BCN, and recently shunned MAN in choosing six British Isles airports to link with ORY. Finnair's normal (non-covid) schedule is upto two flights per day. That leaves Qatar, Cathay and RAM (suspended) who would probably have no wish to use T3 anyway. So ONEWORLD - whilst valued at MAN - is well down the list in volume terms. RYANAIR IS NUMBER ONE, providing multiples of the capacity ONEWORLD does. They need to be front and centre of MAN's thinking.
In T3 and T1, MAG must plan their operations first and foremost around the requirements of Ryanair and EasyJet (TUI and Jet2 are T2 operators). The relatively small Oneworld operation should not in any way be prioritised over them. Personally, I have long advocated merging T1 and T3 at the earliest opportunity. It is indeed just a dividing wall, and an integrated security and immigration system can't come soon enough. This would allow for the Ryanair based fleet to overspill on to some traditional T1 stands where needed, and the flexibility would be a positive for EasyJet too. This would also offer MAN the breathing room to allocate sufficient slots and gates to accommodate Ryanair's full proposed programme for S22, which is difficult to do whilst a significant proportion of T3 is set aside for FlyBe 2.0 ops which may never materialise.
In the longer term, I would advocate using combined T1/T3 as a dedicated LCC terminal with Ryanair and EasyJet the core users. Potentially Wizz too if erstwhile proposals for an initial 4-unit base ever come to fruition. Some of the walking distances to gates from legacy T1 security would be longer than we've been accustomed to, but no more so than at airports such as DUB, AMS and CDG. BA-branded ops would be far better served by relocating to T2 ASAP, and the smaller programmes of IBS, VLG and FIN could easily be moved over as well. QTR and CPA are in T2 already. RAM has not resumed MAN ops, but there is no logic in them being allocated T3 should they return. Aer Lingus Transatlantic schedules which codeshare with BA also operate from T2 already.
Briefly addressing your other objections, I have no issue with bussing where required (and never implied that). Terminal stands are preferred, but bussing is necessary to enable efficient use of resources at times. However, any notion you have that towing aircraft to and from T3 on a regular basis would be a workable long-term solution is absolutely wrong. This would frequently conflict with taxiway traffic flows, causing a major logistical headache and constant delays. An element of towing is inevitable (ideally during the night), but it must never be considered a go-to solution for regular daytime turnarounds. If in doubt, ask LHR.