Implement PSBTv2 field merging

diff --git a/src/psbt.cpp b/src/psbt.cpp
index d03ee404..f01d915f 100644
--- a/src/psbt.cpp
+++ b/src/psbt.cpp
@@ -44,12 +44,19 @@ bool PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt)
     if (!this_id || !psbt_id || this_id != psbt_id) {
         return false;
     }
+    if (GetVersion() != psbt.GetVersion()) {
+        return false;
+    }
 
     for (unsigned int i = 0; i < inputs.size(); ++i) {
-        inputs[i].Merge(psbt.inputs[i]);
+        if (!inputs[i].Merge(psbt.inputs[i])) {
+            return false;
+        }
     }
     for (unsigned int i = 0; i < outputs.size(); ++i) {
-        outputs[i].Merge(psbt.outputs[i]);
+        if (!outputs[i].Merge(psbt.outputs[i])) {
+            return false;
+        }
     }
     for (auto& xpub_pair : psbt.m_xpubs) {
         if (!m_xpubs.contains(xpub_pair.first)) {
@@ -58,6 +65,20 @@ bool PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt)
             m_xpubs[xpub_pair.first].insert(xpub_pair.second.begin(), xpub_pair.second.end());
         }
     }
+    if (fallback_locktime == std::nullopt && psbt.fallback_locktime != std::nullopt) fallback_locktime = psbt.fallback_locktime;
+
+    // Set m_tx_modifiable only if either PSBT had it set
+    if (m_tx_modifiable.has_value() || psbt.m_tx_modifiable.has_value()) {
+        // In general, we AND the modifiable flags
+        std::bitset<8> this_modifiable = m_tx_modifiable.value_or(0);
+        std::bitset<8> psbt_modifiable = psbt.m_tx_modifiable.value_or(0);
+        std::bitset<8> final_modifiable = this_modifiable & psbt_modifiable;
+        // SIGHASH_SINGLE Modifiable (bit 2) needs to be bitwise OR'd
+        final_modifiable.set(2, this_modifiable[2] || psbt_modifiable[2]);
+
+        m_tx_modifiable = final_modifiable;
+    }
+
     unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
 
     return true;
@@ -402,7 +423,7 @@ void PSBTInput::FromSignatureData(const SignatureData& sigdata)
     }
 }
 
-void PSBTInput::Merge(const PSBTInput& input)
+bool PSBTInput::Merge(const PSBTInput& input)
 {
     if (!non_witness_utxo && input.non_witness_utxo) non_witness_utxo = input.non_witness_utxo;
     if (witness_utxo.IsNull() && !input.witness_utxo.IsNull()) {
@@ -434,6 +455,11 @@ void PSBTInput::Merge(const PSBTInput& input)
     for (const auto& [agg_key_lh, psigs] : input.m_musig2_partial_sigs) {
         m_musig2_partial_sigs[agg_key_lh].insert(psigs.begin(), psigs.end());
     }
+    if (sequence == std::nullopt && input.sequence != std::nullopt) sequence = input.sequence;
+    if (time_locktime == std::nullopt && input.time_locktime != std::nullopt) time_locktime = input.time_locktime;
+    if (height_locktime == std::nullopt && input.height_locktime != std::nullopt) height_locktime = input.height_locktime;
+
+    return true;
 }
 
 bool PSBTInput::HasSignatures() const
@@ -505,7 +531,7 @@ bool PSBTOutput::IsNull() const
     return redeem_script.empty() && witness_script.empty() && hd_keypaths.empty() && unknown.empty();
 }
 
-void PSBTOutput::Merge(const PSBTOutput& output)
+bool PSBTOutput::Merge(const PSBTOutput& output)
 {
     hd_keypaths.insert(output.hd_keypaths.begin(), output.hd_keypaths.end());
     unknown.insert(output.unknown.begin(), output.unknown.end());
@@ -516,6 +542,8 @@ void PSBTOutput::Merge(const PSBTOutput& output)
     if (m_tap_internal_key.IsNull() && !output.m_tap_internal_key.IsNull()) m_tap_internal_key = output.m_tap_internal_key;
     if (m_tap_tree.empty() && !output.m_tap_tree.empty()) m_tap_tree = output.m_tap_tree;
     m_musig2_participants.insert(output.m_musig2_participants.begin(), output.m_musig2_participants.end());
+
+    return true;
 }
 
 bool PSBTInputSigned(const PSBTInput& input)
diff --git a/src/psbt.h b/src/psbt.h
index 083cfa53..c5bbff89 100644
--- a/src/psbt.h
+++ b/src/psbt.h
@@ -323,7 +323,7 @@ public:
     bool IsNull() const;
     void FillSignatureData(SignatureData& sigdata) const;
     void FromSignatureData(const SignatureData& sigdata);
-    void Merge(const PSBTInput& input);
+    [[nodiscard]] bool Merge(const PSBTInput& input);
     uint32_t GetVersion() const { return m_psbt_version; }
     COutPoint GetOutPoint() const;
     /**
@@ -956,7 +956,7 @@ public:
     bool IsNull() const;
     void FillSignatureData(SignatureData& sigdata) const;
     void FromSignatureData(const SignatureData& sigdata);
-    void Merge(const PSBTOutput& output);
+    [[nodiscard]] bool Merge(const PSBTOutput& output);
     uint32_t GetVersion() const { return m_psbt_version; }
 
     explicit PSBTOutput(uint32_t psbt_version, CAmount amount, const CScript& script)